it could be several things, but the clutch is where u would start. it only takes about 5 minutes to get the engine out, and clutch off for inspection, so check it. may just be really glazed up in there. before doing this - u can also do a quick check of the diffs by starting the car and applying the throttle. Look at the spur gear. does it spin fast, and the wheels do not. or does the spur gear and pinion gear stay almost stationary while the engine revs? this will help pinpoint if the problem is in the clutch or the vehicle itself.