It's possible that there is some difference in driveline stress that is causing it to do it in full-time versus part-time. It might also be the difference in traction conditions where you use PT that is somehow affecting your CV joint differently.
Just
Empty
Every
Pocket
CV joints can be very fickle; I had a 99' Taurus that had a CV screech in hard right turns until I replaced the tires, after that it never did it again. Get yourself some bigger tires to go with that frankenlift and see if it goes away!
