I need to pull the tcase out soon, once I do I’m going to add the brace back in. I’m secretly waiting for a ORD trans crossmember to match the engine.I wonder if your front driveshaft is pushing and pulling enough on the front output shaft that it's walking the case around and making the bolts come loose. Do you have the side brace from the T-case up to a bellhousing bolt? If not, you should. If it's coming loose when you use 4wd but not when you're using 2wd, that'll be a good clue. That front driveshaft is still plunging when you're in 2wd but with no torque to the front end it'll have less resistance. Don't give in to the urge to brace stuff to the frame. Tie the drivetrain together better.
I don’t think the driveshaft is pushing and pulling, we had it built with the links in, but possible.The frame flex is really bad behind the cab so it twists that area a lot and with my spirted driving style it doesn’t help.