I suspect that your hubs are not unlocking - My 92 Amigo with Asian autolock hubs sticks in the lock mode when it is VERY cold outside (-5 C or colder) and the vehicle has been parked and then I shift back into 2WD when taking off in the cold vehicle. On my vehicle I suspect that it is a grease in the hubs(too stiff when cold) problem. I have learned to shift back into 2WD (and changing direction to unlock the hubs) prior to parking the vehicle. If I forget to do so, on occasion I get the grinding noise followed by a bang that definitely can be felt thru the seatback. The bang usually happens within about 100 m / yds. I never have a probem with the hubs not going into lock mode from 2WD to 4WD.