99 gmc all the lights on 4 wheel drive control are lit up

Are there any lights lit on the dash when this happens ?
SERVICE 4WD: , OVERSPEED , ABS: etc... To eliminate possible problem with the electronic control modules you should have the vehicle checked for DTC'S diagnostic trouble codes in all modules on the vehicle . A code reader for OBD2 codes will not do it .You would need a professional grade scan tool ! Without seeing first hand , it sounds like a problem with the stability control or traction control ,ABS ! I suggest you take it to a ASE certified repair shop that can check it out !

If it has the push button controls on the dash I have had those fail frequently. A lot of times you may push the button and the light may blink and then still stay in 2wd. That switch is also the control module for the 4wd. If it is the know select you need to check for power at the actuator on the transfer case.

You need to have the drivetrain controller scanned for codes, then correct the fault.

yes theres a fuse inside the vehicle in the fuse panel located on the driver side

mine was doing the same thing i found a vacum leek fixed it now it wirks fine

might you have a short ?????

check fuse and relay to switch if power to switch retest switch new doesnt mean not defective if no power check connections and wiring for damage or breaks

Have you gotten a service light? Sometimes these electronic things just freak out. It might be as simple as a new switch. There is a motor on the transfer case that is responsable for moving the levers in the case that use to be done by a lever from inside the truck. I think it may be possible to remove this motor and replace it with a lever. I would check into this by calling a local 4x4 shop, or a mechanic that specializes in 4x4 vehicles.

you could have problems in the shift control module.or faulty switch in dash.

transmission has to be in nuetral to go back out of 4low