U-joint free play - how much is OK?

[Steve Shoyer <Steve@shoyer.com>]

I was under my car yesterday and noticed that there seems to be a lot of
free play in the rear u-joint.  I can turn the driveshaft back and forth
with my hand about 20-30 degrees before feeling any resistance (and hearing
the "clunk" that I hear when I drive).  There doesn't seem to be any free
play in the front u-joint.  Is this OK, or should I replace it?  I'm
guessing that it needs to be replaced, but I figured I'd ask before ordering
the part.

Thanks.

--Steve Shoyer (1980 MGB)