I have a 150 mark iv that is makeing a noise in the track drive when its in mud ,It almost sounds like a spline jumping .WE put the bucket on the track giving it a little pressure and power the track you can hear it but sprocket doesn't turn .It move fine normally you would not know anythings wrong.I have never been into a drive but we do most of our own work.Can anybody give me an idea how bad and expensive this fix might be.Is there a coupler between the hydraulic motor and the final drive that could be getting tired?