I'm an old-school 4×4-er. But not one with cash to burn...
My friend with an old Suzuki Samurai turned me on to locking diffs. I laughed at his little toy "truck" when he got it, but it would go where no others would...climb 50° hills up ATV trails where my Nissan pickup could not even squeeze between the trees!
When he first got his Detroit Locker in his rear diff, he took me on a stream crossing where the exit was a 3-foot high wall of mud. Climbed right out, in "2-wheel-drive". I knew my truck would have to work hard in "4-wheel-drive" to climb out of that.
The thing commonly mis-understood or overlooked is that a 2-wheel-drive vehicle is really a 1-wheel-drive vehicle at any given time. Get stuck anywhere, mud, sand, snow, and the tire on the drive-axle with the LEAST resistance gets ALL the torque. A common "4-wheel-drive" vehicle is only a 2-wheel-drive vehicle at any given time, and, again, the two tires that have the least resistance get the torque. (an AWD vehicle like a Subaru may be different, but they are NOT a replacement for a 4WD vehicle, but that's another post)
But put in a locking diff, and your "2-wheel-drive" vehicle now has the same number of drive tires as a common "4-wheel-drive," except that one of the tires may actually have traction. For simplicity in this case, I am referring a rear duelly pair as a single tire. When you consider that pair to be two tires, then you actually have 3WD vs. 4WD, and a rear-differential locker gains you MORE traction than a setup with two axles with open diffs...
Additionally, a standard differential, is almost always applying torque in a way that is not parallel to the direction of travel. So important to keep your steering wheels strait to get out of mud, sand, snow! With a locking rear-end, the torque pushes you in the direction the wheels spin. It may be minor, but in mud, especially, it can be the make-it or break-it point.
I put a Detroit Locker in my 1984 Nissan 4×4 King-Cab (actually, I had to get a 1993 Toyota V6 4×4 rear axle assembly to get a Detroit Locker, as none were available for my Nissan. The original Nissan L4-motor axle was as stout as the Toy V6, the Toy L4 axle was a skinny little thing! The new axle was 2" wider, though, so I installed 4"-wide rubber wall-to-floor trim as fender-well wideners - I could rub against trees without them breaking off like hard-plastic factory ones would!) and once I got used to it, I feared no road, and none stopped me. Many stopped me before hand. I had always wondered "how the hell did they climb up that slide, even when dry?"
In the pickup, it made driving on pavement a bit different, though. The rear tires would "chirp" when I turned a corner, unless I had a good 1000lbs in the bed (my tools). You could feel it handled differently (no pun) at parking-lot speeds; but on the highway, it was smooth, like an open diff.
So my 2¢ worth is that a locking rear-differential is not merely a first step, but the best step. Some day I'll get mine in my bus...
As for tires, I always ran either mud tires or all-terrain tires on my trucks. I found the best wear, traction, and durability with BF-Goodrich. (Goodyear MTs had no traction in comparison, and did not last). However, I never got more than 20K miles on a BFG mud tire that has no mileage warranty and that costs more than the same size Michelin (same company in reality) or BFG highway tire with an 80k mile-warranty. And the same goes with the BFG-ATs that I never got more than 30K out of. But they would typically "bend nails" down to the ground as they drove over them.
The BFG-MTs were only almost imperceptibly less sticky on dry pavement than a highway tire, but the BFG-ATs stuck like glue. Off-road, the BFG-MT tire was(still is I guess) far superior to others, except for sand - the BFG-AT was(still is I guess) best in sand. The MTs quickly dig holes in sand, where the idea is to "float" on the top - a wider tire with street-tread (aired down) can be better. When I got stuck in sand at the beach 20 minutes from pavement in southern Mexico trying to hide from the Federalies who were robbing us, I had to completely unload my truck (bed full, cab full, another 150lbs on the roof) to get out. No one was going to come tow me outta there, and my friend was crippled from his rodeo days of riding bulls, so he was not much of a stuck-vehicle-pusher! Other times you want to add weight to gain traction. I had traction...too much! It takes very little energy to move a vehicle forward on rolling wheels. Dig yourself 1" deep, and that bump takes so much more energy to overcome... Most Sand-traps will never yield to harder ground beneath - tread across like a feather and stay on top!
Combine all that together and you get a bus with a Detroit Locker in the rear driving BFG-ATs (they last far longer when not used for steering), and Michelin 80K highway tires on the front. A weekend of work and maybe $1500(? - plus labor) vs. whatcha gotta do to add a driveaxle to to a bus.
But given time and cash, a bus with a full-time duelly rear axle with a Detroit Locker; and an on-demand-driven-front axle with either: an Air-Locker (if they still make them) that locks on demand, or my preference: manually locking/freerunning hubs (could you even get them? check Warn Hubs) on a front axle with another Detroit Locker - that is bomber-proof. All that would be doable without too much hassle on a Chevy or Ford cut-away.
Stay away from "full-time 4WD" type setups that waste energy when not in use and typically wear out their components even when not being used. My mom's 1998 Nissan Pathfinder had full-time on-demand 4WD; when in 2WD, the whole front axle was still spinning in an open differential and it started leaking oil. The only way was pull a lot of stuff to open it up and re-seal it. Instead I installed Warn manual locking hubs, so the axle would not spin when not needed. That stopped the leaking, was done in under 2 hours, and cost $200 vs. who knows in parts? Far less cost than shop/labor prices for those projects.
Then there was a client of mine with an old Isuzu Amigo. While installing her front break rotors with integral hubs, I think it was, she handed me the locking-hub rings someone else had removed before she bought it. I re-installed them, which re-engaged her 4WD ability. All was well, until a highway trip. When her front diff heated up, since it was then spinning with the front wheels even in 2WD mode (they were not free-running hubs, same as the factory hubs on my mom's Pathfinder), it started to bind like a locked diff, not an open diff which you
need to steer on pavement. So when she would try to change lanes, the vehicle would suddenly and sporadically pull to one side. She stopped at the local (**only**) vehicle repair shop and they told her it was the brakes I just replaced. They **only** replace the enter break system, front-to-rear, including all steel brake lines, at that shop, and quoted her another $2000. I removed the lock-ring she had given me, for free, and she was fine. If it was her breaks, it would only happen when applying her breaks, not when steering. Those goons never listen to anything you say except how much you say you can afford....
Bottom line is....keep your front differential free from use unless you are using it.
Aloha y'all
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