I just got done replacing the transfer case on my Dad's 2000 Expy because it was stuck in 4Hi. After it was done, I took it on a short drive around town and everything seemed ok, it drove fine and the 4WD finally worked. The problem is as soon as I took it on the highway there was a shudder that felt like the truck was trying to shift into 4WD. I pulled the fuse for the A4WD and took it on the highway again and there was no more shudder. Any ideas? Could one of the sensors for the A4WD be messed up? Is there anyway to test them? Or could the control unit have problems?