A few more points about the 4R100 trans.
Many burned out 4R100s got that way when the engine was overheated which then overheated the trans fluid. An add-on trans cooler can head off those problems. That and not overheating the engine in the first place.
Most of the reliability problems with this trans came from the F series pickups, not the E series vans. The van engine room is too small for a combustion air intercooler so the 7.3L diesels in vans have been de-tuned to compensate. The F series pickup motors have more horsepower (also better fuel mileage) and pickup owners are way more prone to modify their diesels for more horsepower and then use that horsepower at every opportunity. All this puts more stress on the pickup trans than the average van transmission will ever see.
Obvious warning signs are brown, burnt smelling trans fluid and, if you drop the trans fluid pan, metal bits in the pan and around the magnet inside the pan (you have to drop the pan to see the magnet).
If there are problems with the 4R100 they can be rebuilt to a much higher torque rating relatively inexpensively (Yes, I know that nothing transmission related is ever truly cheap). A properly rebuilt 4R100 is often better than a new one and way cheaper in the bargain.