have you tried taking the motor/pinion away and just spinning the spur to see if you can feel resistance/binding?

As above take parts away and try different configurations until you find it loosens up.

I think its the univeral joints in your driveshafts. If the pins are even slightly out of line they will click as they go in/out of the cup.

Please could you tell me if you are running Durango wheels dyed black or another wheel?

I have tried Losi wheels on the back and the are a slightly different offset causing the wheel to rub on the rear camber link on full droop.

They are durango wheels dyed black:thumbsup: