My 96 S10 takes a long time to switch to 4 wheel drive. I can push the button and hear the acuator move and the light will say 4WD but it won't be in 4WD. Maybe about another mile down the road you will hear a big thunk and then it will be in 4WD. Been to the dealer 3 times and he says it works fine. WHere do I start looking for the problem?