Say I think it is the rear diff. Hopefully you can just shim it. UMM, go to the Hpi website and download the first nitro mt manual, look for the diff assembly and you will find the part number for the shims and see where they go. When you tear down the diff inspect your gears they may or maynot be stripped. When I did mine I found that the gears that everyone else was stripping were not the same gears that stripped on mine. I shimed mine and it works great but I failed to put new gears in mine so its just a matter of time before I have to replace the gears that were stripped. Hope I helped!