Well, to start with, you should never run the vehicle on hard tractable surfaces in 4wd with the hubs locked. That was or could be the start of some very unwanted problems. The reason being that, while you are turning, each wheel is wanting to turn at a different rate of speed and will cause the drivetrain to bind . Even going straight can be bad unless all four tires are exactly the same size by tread wear and inflation which is nearly impossible to achieve and hardly worth the effort since you do have a 2wd option. I hope the cracking/popping noise you heard wasn't your birfs letting go considering your gear ratio and assuming you have large tires.
If I'm correct the tru-trac unit is a gear driven limited slip differential which will allows limited differentiation of wheel speed per axle in a turn and will only engage at a certain speed durnig wheel slip. I'm not familiar enough with that unit to know of a way to statically check it other than to remove it and see, someone else here may be able to provide that info. If anything, it will also cure your curiosity for whats actually in the rear end and whether or not there is any damage. You say the size of debris is about the size of sand or could it be more like a dark wet powder? If it is more like powder and no real chunks can be felt between your fingers as you rub them together than I would say this is normal wear from the gears and not be worried about it. Anything bigger could be reason for concern especially if you start to hear unusual noises. Also did you buy this truck like this? Were you told it had these diff's in it? Is this why you're not sure about the tru-trac's?
As far as your rear end goes it sounds like the brake adjustment is causing the brakes shoes to lightly rub on the brake drum as you turn the wheels, this is perfectly normal! The ticking noise could easily have been the diff side gears as they walk around while turning the wheels, also normal. Hope this sheds some light on what you're looking for.
