To see if it is the 4WD indicator light on the dash do the following:
1) push and hold the trip meter button
2) turn on the ignition while holding the button in
3) release the button and all of the lights on your dash should cycle (one at a time)
4) watch for the 4WD light
If your 4WD light works, the next step is to replace the 4WD switch on the transfer case.
from what i see on here all the time this is a very common issue with a TJ. it happened to mine. there was no cleaning it or anything it just took a crap. and before i take it out and try a quick fix i would spend the 20 bucks to fix it right. if it was something more expensive i might try something like that but from what i have leanrd in the past tho quick fixes never last long