I have a AMC 71 AMX with a bad vibration . It actually cracked the bell housing of my 904 tranny . The motor and driveshaft were balanced , all new mounts , I'm using a 8 3/4 rearend and that's the only possibility left . There seems to be a lot of play in the pinion . The symptoms your describing sound very much like my problem . Do the Mopars with Dana 60 rearends have the same problems with vibration ?
