U-joints?

Ive been test driving some xj's year 1996...i noticed some of them have this single "slapping" sound coming from underneath whenever in motion, say your cruising at any given speed and your just coasting, if you push the gas pedal in for a split second to accelerate and then let go immediately i hear this "slap" type sound when i hit the gas and then once when i get off the gas, i was thinking sloppy u joints but i can never judge if they are bad by twisting the d/s by hand b/c i dont know whats normal and whats loose. What do you think? Thanks for any opinions.

I'd like to know about this too... My 99xj (87k) has the infamous "thud" when going into reverse. Motor mounts are fine, so I was thinking maybe the rear driveshaft ujoint was the source.

I believe that no tolerance is good tolerance - in other words, no play at all. At least in the driveshaft/UJs.