I replaced my front DS ujoint, where it met the axle, and now it seems to be going out again. I am starting to speculate that it is one of my rear, of the DS, two ujoints. Is there a way to check which it is or should I just replace both?

I am getting vibes and it is extremely loud on the road.

 

did you grease the new ujoint? try greasing all of them if they have a fitting, and the ds itself and go from there. you can check for play in the ds by seeing how much it turns.

 

just grab the yoke on the axle with one hand and grab the driveshaft with the other and twist in opposite positions. There should not be any movement, if there is, replace. But i agree try to grease everything first.

 

same as above, but you want to block the wheels and put the jeep in neutral, this relaxes any bind hold on the joints.
you can also push up on the driveshaft at the u-joints for any play.

 

Each side of the joint held in either hand (joint between hands.) For a double Cardan, you can grab the H-yoke to hold one side.

Twist hands in opposite directions, note play (should be none.) Try to separate shaft along each axis of joint, note play (should be none.)

If you note just a bit, grease joints and check again. If play is still noted, replace all joints in shaft (replacing one joint can lead to premature failure of the rest - may as well get them all while you're in there. If one joint is shot on the rear shaft, replace both. One in the front? Replace all three. There's only one joint in each shaft on the front axle, behind each front wheel.)

 

Just went though this on my '98. It was more of a thumping noise than a vibration. I couldn't feel any play at all in the upper or lower joints. Finally pulled the DS and it would bind in the center ball between the upper two u-joints. Local drive line shop want $250 to rebuild. NAPA had the parts for $110. Tom Woods had complete new ds, built to your measurements for $270 delivered in two days. Fit, finish, and apparent strength is great.