bad driveshaft on V8?

["Jim Dhillon" <jdhillon@cibc.onramp.ca>]

I get a thump in the back when my tranny shifts occasionally on my V8..
could this be due to a bad u-joint on my driveshaft? I reached under my car
and turned the shaft back and forth by hand.. there seems to be some play
before the gears start turning on each end. Any place else I should check
before digging into removing the exhaust and driveshaft for a looksee?? 
How can one diagnose a bad CV joint? I get some vibration at highway
speeds. 
Also, when I drive over a speedbump, I feel a bumping noise as if something
is not quite tight. Any ideas on what might be the cause - it feels like it
is in the front end somewhere but I have checked wheel bearings and tie rod
ends. All seems ok.