I got under my car the other day and the first thing i noticed is that my differential was caked with oil, new and old. Would all this oil being on the OUTSIDE of the differential as opposed to being inside where it belongs cause a "Clunk"?

Unlikey but you could always do something like, drain the diff oil and refill, drive it and listen for the clunk.

Clunk is normally attributed to either a faulty diff mount insulator, arrestor strap, half shaft or drive shaft universal joint and or a combination of all.

clean off as much of the gunk as you can (Simple Green is a great product for this) and open the filler hole. If you can feel oil at the lip of the filler, you are good to go, though it might be a good idea to drain it anyway-who knows how old it is!

I got back under my car and the U-joints all had a thin layer of rust on them. I know the actual U-joint is only $10-15 each, but how expensive is it to have a shop replace them for you?

U-joints are pretty easy to change. With shop rates near $100./hour do you really want to find out? You need a few basic tools and it would cost less than paying a shop. (Hammer, socket, snap ring pliers and two 12mm wrenches) To be safe I always use red loctite on the driveshaft bolts when reassembling.

Check for loose mounting harware by tightening all harware related to the diff. Two common culprits are the mustach bar outer and inner nuts.

2c

Jim