I was watching the movie Roller Ball last night and noticed a dual axle XJ in it. Has this ever been done before?

Roller Ball, now there's a classic

like two back axles? i have seen it on toyotas and stuff and it looks funny. never on a cherokee can i say have i seen one. probably take lots of body work to make another well. or are you talking like a dually pick up with two wheels on each side in the back? i know they make a bolt in piece that lets you run dual wheels on each axle for most vehicles and you can buy the kit on ebay or probably anywhere

Yeah i'm talking about two back axles

They cut the back and extended it for the new rear well and axle. Plus a ton of bodywork, to make everything look correct. (just like how they make limousines)
And for the movie, Im sure that 2nd rear axle isnt being driven.

Although, with enough money anything is possible, and you could make the 2nd rear axle drive
It would be kinda pointless though, not even enough of space too load it up to its new capacity rating...or...not enough power too pull its new tow rating...

So its just for looks and pretty pointless.

You can make one of those, it takes a lot of good fab work to make it look right though, and you'll need custom through-drive axles.

Actually, you don't. 6 wheel drive Jeep CJ6s were not common, but there were a few around. You need to mate the parts from a D18 and D20 tcases and you have 6 wheel drive capability.