My jeep makes this loud noise when you try to drive it. I had it look at they told me at first I need a transfer case. But I put some oil in it and stop. Now it's making some louder noise. When you step on the gas it feel like something is wrong with the gears. I was told that I all I need was a U Joint then some one told me that it could be the differ can some please help me please.

could be a bunch of possiblities take to reputable tech and have them check it.