98 EB 4x4 Expedition-Need help-when driving at speeds of 55+ and need to turn steering wheel for a curve the steering wheel and front of the truck starts to vibrate so hard that it is hard to keep control of the truck. By backing off the accelerator and sometimes braking I can get it to stop shaking but sometimes the only way it stops is when I get to a straight area. It appears to only do this when a curve goes to the right. I've had the tire rods checked, replaced the ball joints. I'm begining to be concerend that perhaps the 4 wheel drive is kicking in when it should not. THe light doesn't come on. Has anyone had a problem with the 4-wheel engaging when it's not suppose to be?

I'm taking it in again to the mechanic and we're putting it on the lift to look again to find out if we've missed something. but this has me really concerned.