What's wrong with the 4wd, we can probally just tell u what to look for. A common problem with the yj is the 4wd vacuum acuator on the front axle towards the passenger side. It has 2 vacuum hoses going to it. They cost about $100 new, if its bad you can easily replace it or switch to a cable. If your 4wd light isn't coming on inside when u engage it, that's probally what it is.