How many miles are on your TJ? Does the slippage you're feeling happen only in 4Hi or 4Lo?
It could be that the steel drive chain inside the transfer case has stretched enough that is slipping... especially if it has some miles on it. If so, that's not an expensive or difficult repair. It does involve opening up the transfer case but there's not too much to a transfer case... it is nowhere near as complex inside as a transmission is.