+1 on what's already been said...with a caveat. The U-joints, pump away. Push out the old and in with the new.
The slip yoke I went ahead and dropped the shaft last time (I was working on the t-case anyway). I marked it, pulled it apart, smeared grease on the splines, and put everything back together. It didn't take long. There's a pretty big cavity where the two fit together. I'm not sure how much grease that will hold but it's a lot.
However, if you've ever done the "pump till it's full" it acts like a big piston. You can actually watch as the two parts of the driveshaft are pushed apart (slightly). Much like in other hydraulic pistons, the liquid doesn't compress and has to go somewhere. So lets say you fill that cavity slap full of grease and now the driveshaft can't compress. Then right out of the gate you decide to go hit some huge bumps. All that rear axle movement is going to be transferred right up the shaft and into your T-case. Not good.
After you drive a day or two and let the grease and drivetrain heat cycle it's going to push/sling out a good part of that lubricant. This leaves free space again for the splines to move in/out. In a nutshell, that's why the "clunk" comes back. If not, you'd be able to grease it once and be done. Long story short...don't pump the driveshafts full of grease then immediately go run the Baja 1000 or try to catch air on the parking lot speed bumps :cowboy:
