I am trying to chase down a vibration in my driveline that only occurs while coasting off the throttle above 30-35mph. I have check the u joints for "play" but not entirely sure if I am checking them properly, what is the correct method for checking ujoints front and rear?

you want to remove any driveline bind, so on level ground, put the jeep in neutral. use the parking brake if it works, if not, be sure to block the wheels.

then get under the jeep and grab the driveshaft with your hands before and after the u-joint, and twist to feel for any play. if there's play, replace.
but, sometimes the u-joint is still tight, but seized, so you may have to remove the shaft to feel for the seized joint. if it doesn't move, replace.

same way for the front axle shaft joints.


btw, sounds like your rear driveshaft u-joint. check the rear most joint in your rear shaft. the one near the pinion. most likely culprit.

cheers.

Thanks for the help! Will be checking my rear ujoint ASAP.