You need to have the vehicle scanned for trouble codes.
Who ever scans it must have the capability of pulling "C" codes.
If you do a lot of mudding, then water may have gotten into the wiring of the speed sensor.
If the trouble code is DTC C0374, then have your mechanic check and perform TSB Bulletin No.: 05-04-21-003A
"Service 4WD" Light Illuminated, DTC C0374 Set (Inspect Wiring Harness to Transfer Case Speed Sensors, Replace Wiring Harness)