4 wheel drive

L have a 1993 GMC Jimmy. When l push the botton for it to go into 4 wheel drive the lights flash and then l hear a buzzing noise around the glove box. But it does not go into 4 wheel drive. I took it to a mechanic and he did a computer scan and said it was the actuator motor on the trasfer case. So we changed it and it still doesn't work.

What is wrong with it and how do I fix it?
Is there actually a problem with the actuator motor or is it something else?

Thanks,
Jack