Well, typically an unbalanced tire will seem to cause a shaking feeling, but a u-joint will be a much faster or more high-speed vibration, probably more like a humming or thrumming feeling (and sound) in the vehicle.
BTW the difference is (IMO) because the wheel/tire is a large rotating mass, so it tends more to create a wobble you can feel. But a u-joint (or cv joint) is much smaller & lighter, and spins at a faster rate than tire rotation. For instance, if you have 3.42:1 axle ratio, the driveline will rotate 3.42 times for each tire rotation. So the smaller, lighter, more rapidly rotating mass will cause more of a hum, less of a wobble.
Kind of hard to describe but I hope this helps.
1995 5.2 ZJ Sold... but still in the family