The above method doesn't always work. I LITERALLY just got done replacing a ujoint on my truck this morning. Mine had no slack or wiggling. In fact, other than weird noises and slamming/clunking while up shifting or surfing the gas, I couldn't tell the ujoint was the issue. My entire driveshaft had absolutely no play in it.
sold me a brand new ujoint for $10. If you have a vice (or better yet, a press) the job will be easy. If not, it will take a couple of hours and some ingenuity. My neighbor and I used a pipe-wrench as the vice!
Mark up where the driveshaft meets the transmission and rear-axle (scratch it, use hot pink nail polish....). Drop the driveshaft (4 nuts at transmission, 4 bolts at rear axle, 2 bolts for the carrier bearing). Drag that heavy bitch into your garage and start articulating all of the ujoints, nOting which ones don't move smoothly or at all. If you notice any physical damage with a visual inspection, replace those as well. Spin the carrier bearing, feeling for slop or roughness along it's orbit.
To extract the ujoints, well, let me know of you get this far.