If it pops out of 3rd gear there are two things I would check right off the bat. It could be as simple as a shift fork adjustment. Not done properly, the fork could be pushing the 3-4 shift hub away from the 3 side, toward neutral, not allowing it to engage properly. The other thing could be the back cut on the synchro hub teeth are all worn. That causes the gear to not stay engaged. Full disassembly for the latter, but the gear stack needs t be pulled in either case, which is most of the work.
Do this at Carlisle? You first, not for me. I'd need a bench, tools, and a shop press.
Interesting proposition. Does it have long or short axles?? I'm interested in short axles.
I actually have 2 IRS spares. What I really need is short axles and tubes for spares. Then I could build up a nice custom geared spare for myself.