I just rebuilt a Saginaw box for my Iron Planet purchase. That was the only major defect on the HMMWV, other than a lot of small piddling issues like wipers, wiper sprayer, missing lights, etc. It was not that difficult to rebuild. Hardest part of was getting it off the vehicle. If you can find a way to test it before putting it back on, that would help even more. I know they make a test machine for these things, and that would have saved me some redundant work on mine.
I rebuilt my box, re-installed it and instantly the low-quality pitman seals failed. I found you can get a better seal at the local parts store for $10, then replaced it while still on the machine and it was a relatively easy fix, so if pitman seals are leaking, you don't even have to take the box off to fix that issue on that Saginaw box. That was about a 30 min repair.
However, once I got that pitman seal replaced, the low-quality seals I originally had failed at the input shaft too, and that one did require me to remove the box again. If you didn't mind removing the generator, you could probably replace that seal as well without removing the box from the HMMWV. I didn't want to fool with the Generator, so I just removed the steering gear box and replaced those seals. I was glad I did it that way after all too, because the retainer cap O-ring was poor quality too and looked as if it was leaking some as well, so I replaced that with the new and better O-ring that came with my new kit I got at the local parts store. All is working well now.
But yes, I am no mechanic, and I rebuilt my box. The Sheppard box looks much smaller, so maybe easier to rebuild than the Saginaw? I don't know, but don't be afraid to try if you can find the correct repair kits. And if some local repair shop has a test bench for them, pay them $20 or whatever to test it so you know it doesn't leak before reinstalling. It would be worth the costs. If you get good quality seals right up front, you shouldn't have the issues I had though.
On the Saginaw, search out a repair kit for a 1979 Malibu Classic, as that is what I used to find my local seal kits and they were cheaper and better quality than the kit I got from one of the military parts sites. Just be aware that there are two different input shaft sizes, and the one for the HMMWV is the larger input shaft.