4wheel drive wont work

My MJ doesnt want to engage the front axle when i put the case into 4wheel. the little light goes on but the front wheels wont engage. the vacume pod and shift fork all seem to be working fine. Once and a while it will act like it is working, but the second a load is put on it it makes a loud pop and then goes back to not working. Is this a bad hub? if so how hard is it to replace one? thanks

try this.
http://www.naxja.org/forum/showthread.php?t=89861

After you've done the easy part and checked all the vacuum lines, check the shift fork. If there is ANY side to side play in the fork, then it probably cracked at the hole where it slides over the diaphragm rod. Physically take the fork completely out of it and inspect it. It's fooled me before. BTW when you go to replace it, try to find a cast iron one out of an older dodge instead of the stock aluminum one.