If a G3 or G4 look at the steer motor. Double check the prox is working, then you are looking at the steer transistors

Does it do it on start up?
If it passes self test, and does it when running , jack up truck, block etc, and check for play in the drive unit - make sure it is attached to the frame tightly - the mount bolts can break or loosen

If it fails on self test, jack up and before you start teh truck move the drive unit so its pointing left/right.. code may change, probably the transisitors