Might be a U-joint. Just ebcause it was recently serviced doesn't mean it can't be bad. But I have a similiar problem, whenever I am in 4x4 and the truck is just putting around it's fine, but if I put it under load like going up a hill, accelerating hard, or turning the wheels to load up the drivetrain, it makes a loud clickling/grinding noice. I have'nt figured it out yet though, replaced drivesahft u joints, axle joints, thuroughly inspected the CAD, spinder gears, ring and pinion, hubs are fine, everything seems fine. I think it's in the transfer case.