Bad pinion seal

Will a loose u-joint ruin the pinion seal eventually? Also what damage can be done by not fixing this immediately?

The seal will not last long depending on how bad it is. I've seen new seals go bad within 200 miles. :) The next thing that will go will be the bearing and your rear end will then make all kinds of not so good noises and start thumping. That could however take some time but you will need to fill the fluid up a fair amount or atleast check it a lot.

a bad u-joint is much harder on the bearings than the seal. those seals leak, if its not leaking now, it will someday. and if a new seal goes bad(assuming a good/new seal) its because of improper installation.

Well I was able to get under my car yesterday and the u-joint does not have very much play at all. However, the seal is leaking a lot, the pumpkin is completely covered in oily crap. So, is it possible the clunk could just be from low fluid in the rear? I'm going to change the seal and replace the fluid this weekend to see if that helps.

how can u tell that the ujoint is bad?

From what I have read here, a bad u-joint will cause vibration when you drive. Also, if you place the rear on jack stands and twist the drive shaft with your hands there should be virtually no play. Worn u-joints will allow the shaft to move more. Based on this I have concluded that my u-joints are fine. I have no vibration and there was very little play.

I have also just read that my clunk could be coming from a worn bushing. This is most likely since the clunk is most noticeable when I go over a speed bump. On Monday I will report my findings.