I have a 1999 GMC Jimmy and the 4WD doesn't work, It has the control on the dash and it has the automatic 4WD option on the switch. When you turn the key on the entire switch lights up and the transfer case mod makes all kinds of switching noises and the lights on the switch flash and then go out except for the two wheel drive . The switch in the dash was replaced and the controller on the transfer case was replaced and what i noticed is the transfercase is locked in 4WD. The diff isn't locked so the power isn't getting to the front wheels but I can't turn the front drive shaft with the rear wheels on the ground but if you jack up the rear wheels and the car isn't in park i can turn the rear wheels by turning the front driveshaft so that is how i know the transfer case is locked in 4WD. I've checked all the basic stuff like relays and fuses connections but I'm at the point were i don't know what elts to check for and the last mecanic just started guessing and replacing parts, I need to find the problem and fix it.