1998 Ford Expedition 4 wheel drive not engaging

Hi, when I got the vehicle everything worked just fine. But as time went on, and when I needed to use the 4 wheel drive, on the first incident, it would not come out of 4 low, I called the dealer, they had me take the vacuum line off the front drive line, so it would dis engage. Since then, when I would turn the switch from 2 wheel drive to 4 high, the switch would make a clicking noise, then it would finally engage into 4 high, now it don't click or make any noise, and wont go into 4 high or 4 low, yet the indicator light will flash when I put it into auto 4 wheel drive, but wont engage either. I'm lost and could use some help on how to find where the problem is. Thanks. Dano