Like I said, unless you want to spend the time learning how to fix the transmission in that one 4x4, since your not using it and you can't use it right now, you can tear into that tranny and figure out whats wrong. I would honestly do this, then you could have 2 running trucks! And save a significant amount of money in the process.
Worst case scenario you give up and you still have a 4x4 truck that doesn't run. Best case is you get it working right while spending less money and time than the first suggestion of mine. It may look like Everest when dealing with a Tranny, (and I have never had to work on one yet) but from what I have seen, as long as 1) you take your time, 2) you film what you are doing to look back at it, 3) keep things/parts labeled and organized you can do it. It can be scary at first, but everyone has a first time for everything. I guarantee there is someone on this forum that has rebuilt a transmission for these trucks.
You can figure out what failed, get the transmission fresh new parts in the process and use it as a learning process. The transmission is the hardest thing to fix on a car, outside of people that do repairs to ECU/BCM's craps plain Wizardry to me.