Are your hubs auto or manual? If manual make sure they are unlocked. If they are auto get someone to watch your front drive shaft while you slowly drive by in 2wd. If the drive shaft is turning when the t-case is in 2wd then you may have a hub that isn't disengaging. That would cause your front shaft to turn even while you were in 2wd, then when you put it into 4wd it puts a load on the shaft quieting it down. Still check your u joints.
This is just a possibility.
Good luck