04 silverado has "service 4WD"

Quick and dirty - try to disconnect the battery for a short time. Reconnect it and see if your 4WD controls again work.

Have a scanner read the codes, if you want more specific answers. Condition Some customers may comment on intermittent illumination of the "Service 4WD" light. Upon investigation, the technician may find DTC C0374 set. The customer may also comment on intermittent erratic operation of the 4WD or AWD system after driving through rain/snow or simply going through a car wash.

Cause The speed sensor signal may have become corrupted. Possible openings in the speed sensor wire insulation (twisted pairs) can allow water intrusion. Also, wire connections contaminated by water may result in short circuits and erroneous speed sensor readings. This most often occurs on the rear speed sensor circuit.

Correction Inspect the wiring harness to the transfer case speed sensors. On the full-size pickup and full-size utility models, replace the 2.2 m (88 in) pigtail harness that runs from the C151 connector under the hood to the transfer case. The smaller transfer case harness splices into the larger chassis harness a few inches in front of the crossmember. The chassis harness routes along the left side of the frame under the driver door area. Use nylon tie straps to secure the fabricated harness to the main chassis harness between the transfer case Do not over-twist the two wires in the replacement harness.

Check all grounds for poor connection. Also inspect the engine grounds, as other components grounding through suspect grounds can then seek other ground paths and interfere with transfer case operation. Battery cable engine grounds as well as grounds located on the back of the engine cylinder heads and engine block are possible sources of this concern

You can start by pulling 4WD fuse in the panel under the hood. Sometimes it's called "ATC", sometimes it's "4WD". Pull it for about a minute and re-install. If you get your lights back, you may need a software reflash. In any case, guessing is not going to really solve the problem. Vehicles these days are just too complex to be throwing parts at them without even scanning for codes.

Ian

I had that problem also. It turned out to be the 4WD switch on the dash. I removed the switch and had to resolder the connections inside the switch module. I haven't had any problems since I resoldered the connections.