The backup lights on my 98 D 4x4 slt wont work when in reverse. I have replaced all the bulbs and still not working. Anyone know whats wrong and what I should check?

 

Got the same issue on my 98 slt. Don't know what the fix is other than when you put the car into reverse push back up on the shifter towards park. My lights always come on after that. Seems that the indicator is slightly off.

 

Found a good bit of info on this problem.

Believe it to be the Park-Neutral-Reverse Switch. The Haynes book has pictures and instructions for R&R. Couldn't get to the pins to do continuity test at the sensor (they say the outer two pins of the three pins should be "shorted" when the truck is in reverse). This closes the circuit and passes 12volts to the rear bulbs.

Waiting on the part ~$50 to come in.
Have 4x4 and have climbed under truck to see the switch is reasonably easy to replace if you have a 1" deep socket and a small screw driver to get the connector released.
Have to remove the steel plate that protects the bottom of the transfer case (6 bolts with 9/16ths socket).

Last night had to replace the Output Speed Sensor which is also on the drivers side of the transmission and requires removal of the steel plate.

Watch out for Trans fluid when you pull the switch out. Have the new one in your hand and see how fast you can unthread the old one and insert the new one. If you lose less than 1 cup of fluid you are faster than me.

Best of Luck.