I thought with the part time kit you also had to turn off the center diff lock button to allow the front drive shaft to spin freely which should take the load off the front diff. Also make sure the locking hubs are actually unlocked. I had a bronco that would look like it was unlocked on the Warn hubs but would actually not unlock unless there was zero stress on the hub. I had to get the car on perfectly level ground with tcase in neutral and steering wheel straight. The bronco was a total POS so that method may not apply to a well built 80.