pull it apart completely and see if any the teeth are missing or worn down i had the same problem with mine had to replace with the hardened set from the 18ss mt2 and also shim them just to keep the teeth closer together another way to check is hold one rear wheel and spin the other one there should be some resistance not alot and the front wheels should move aswell also do the same to the front aswell.if theres no resistance you got a diff on the way out hope this helps these are the parts you want to check http://www.hpieurope.com/piw.php?lang=en&partNo=86014 and this http://www.hpieurope.com/piw.php?lang=en&partNo=86336