I'm getting a loud clunk from under my 2000 Blazer when shifting from park. There's also some loud squeaking when driving. Would both these problems be caused by a bad U-Joint.

I got the same problem, I've already checked u-joints out for tightness, but can't tell anything. Looks like we both need help. Let me know if you got your's fixed please.

Check this forum out. Might be what your looking for.

http://www.automotiveforums.com/vbulletin/showthread.php?t=302656

Both problems you mention would be indicative of bad u-joints. Not saying they definetely are, just saying that your assumption is correct.

Yip, I concur.