Yes, you can put manual-locking hubs on your truck without changing the t-case. I had it done at Les Schwab in Brookings, OR on a 79 GMC Jimmy about 8 years ago for about 75 bucks. As for getting it back to 2hi, I was having the same problem, (that's why I swapped the Hubs!) try shifting the tranny into neutral first, if that doesn't work try shifting tranny into reverse, back up a couple of feet, and then shift into neutral, and try shifting the t-case into 2 hi. They can be temperamental at times!