Your rear end ratio is 7.16 to 1? Is that right? The best and cheapest thing you could do is to get a lower ratio differential, IMHO. Adding a two speed rear end really won't help you any unless the final ratio changes. Two speed rear ends, from what I understand, are usually configured so that they don't add top end speed, but give you the option of dropping into very low gears (even higher ratio).
I've seen used differentials for IHC products here: http://www.trucks.com/parts_inventor...ort=1&dir=desc
for anywhere from $750 to $1250. You're probably looking at another $750 to get one installed (guesstimate). Total cost from $1500 to $2000. That'd buy a lot of gas or diesel...
I want to do a differential swap someday so I can use the Interstate highways. Right now I'm limited to secondary roads because my top speed is somewhere in the neighborhood of 45-50MPH.
I doubt fooling with the tranny is going to give you any advantage, simply because the limiting factor is the final drive ratio, and until that changes, no amount of (expensive) transmission work is going to make any difference for you. IMHO, YMMV.