Would help a bit to get some info...
You posted in the TJ section so I'll assume thats what you have.
Engine, tires, lift don't make a difference so we'll skip that. You are able to drive it so it wouldn't be your transmission.
What happens when you put it in 4-hi? in 4-lo?
Two ways you could check to see if 4-wheel is even working:
1) put all 4 corners up on jack stands and see if the front tires move (not a fan of this one)
2) disconnect the rear drive shaft and drive in front wheel drive
If the front tires don't go, you have a problem with your transfer case. If thats the case, it is either simply not engaging (a problem with the 4-wheel drive activator linkage) or a problem internally (i.e. the chain in a NP231 has broken).