Got to be in neutral and foot on the brake before it will shift either way. The T-case is completely separate from the front hubs. The light on the dash has no way of knowing what the hubs are doing, only the T case. The hubs are vacuum latched and unlatched. The vacuum is not constant. It sends a pulse of a certain pressure to latch and another pulse of a different pressure to unlatch. The pulse lasts about 30 seconds or so after the dash switch is moved, the pressures are as best I can remember 6” and 15”. There is a solenoid on the fender that regulates the pressure and operates the hubs. All of this system can be easily tested with a hand vacuum pump/tester. Again, if the dash light is staying in 4x4 low then it is something with permissives or T-case actuator, if the hubs will not cooperate and the T-case and dash lights work right then it is the seals or vacuum system. It is a good idea to periodically reach in and make sure both axles turn free when in 4x2