If I am not mistaken the RTR comes with the largest spur HPI makes 52 teeth. The thing I keep going back to is he says it makes a clicking sound! Check the clutch, pull the slipper all the way apart and check it out. Then if you do not find anything go to the differentials. When you did like I said hold 3 wheels and turn the 4th did you use a little pressure on the 4th wheel when it go to point the wheel felt like it would not turn any more. Being I do not have the truck in front of me I am still guessing the diffs as he said about the clicking noise but I could be wrong. If I had in front of me I could tell really quick.
Good Luck