Fred they do not disengage when shifting gears.. I betcha what you are describing is the TURNING of the wheels.
When behind another vehicle and turning left to pass under power the locked wheels do not want to go left. As one will have to go faster than the other.. I betcha that is what you felt.
The only time the locked wheels unlock is when coasting and turning. At that time the locker allows the wheels to turn at different speeds.. BUT when power is applied both wheels MUST turn at the same speed.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />