1989 for f150
Could be a bad transmission, could be busted rear differential ring and pinion. Does the driveline spin when it is in gear? if so then it the rear differential, if not then it's the transmission.
That was for and automatic transmission above. Now a stick shift is slightly different as you have a clutch involved, if the clutch is worn out then it won't move under engine power as none of it is getting transferred to the transmission. Pull the inspection cover off the bell housing and if clutch pieces fall out then you found your problem. You can check to see if the differential turns by blocking the truck and jacking up the rear and in neutral see if when turning the driveline by hand it turns one or even both rear wheels in either direction, if they turn then the ring and pinion is still in one piece.
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