Put your car up on jack stands and pull the hubs away from the axles but leave the axles in the diff. Then try spinning the axles both in gear and in neutral they should spin in opposite directions when in gear. If everything checks out then the axles might be binding in the hubs. If they don't spin then it might be your differential or other internal tranny part. Oh yeah pull the spark plugs before you try to spin it while in gear. Good luck.