Does it pop while going straight? It will make noises while turning, just the nature of an auto locker.
If it pops while going straight under load, take the cover off and inspect it. I'd suspect broken sheer pins. How old is it?
Bought truck used, I've already been in the cover all looked good. works normally in 2 wheel drive but under load, straight and any incline pop...pop....pop like the locker is in and out...
ordered pin kit and heard thrust washer may have been left out??? going to pull apart this weekend just looking for more idears!!!!!!
Could also be a stretched t-case chain skipping teeth on the sprockets. "Under load in 4wd" is a good indicator of that...and feels like it's coming from the front end. Jack the front end up, put it in 4wd, and twist the front shaft back and forth with your hand. If you hear metal on metal inside the t-case, chances are your chain is stretched. I went through this shortly after installing a Lock Right in my first Jeep. Drove me nuts thinking it was in the front end. $65 later I had a new chain in and was good to go.
Not saying that's what is happening here, but it's something to check out.