I've run into u-joints that weren't loose, but had worn into a pattern where when moving the joint it felt like it had a detent in one postion. Not moving smoothly through it's range of motion could give you a vibration without any significant looseness in the joint.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes