First let me welcome you to ColoradoK5.
This board has been running non stop for over two years and most of that data base is still available . In that time EVERY question you could ever come up with has been answer about 50 times.
The fastest way to answer your question is to use the search button. Select all forms and 6 months for the date and I can guarentee if you use the key words Thump, clunk and bang at least 50 hits.
I didn't check your bio but I bet you have a 81 up with a 208 case. They are famous for a bang, clunk. It's the splines in the tail shaft of the tranny. Broken tranny mount will also cause the problem.
Asf for your brakes....I would recomend replacing the shooes. Gear oil contaminates the pads and causes the friction to be different. That will cause brake pull and other irritating problems. Gear oil is also flamable if it get's hot enough so there is the potential that you get thos shoes hot enough to catch fire. Fix the seal and replace the shoes on the rear axle, yes both sides so that you don't run into any brake pull problems.
