Ave you taken the slip joint apart and put the shaft back together?
The joints need to work in opposite phases by 90 degrees to cancel out the change in angular velocity on each joint but there are two possible points and they are only balanced at the factory in one, at 90 degrees or 270 degrees. It can go back together in 20 odd different ways but only one is correct.
Look at your joints, are they 90 degrees out of phase? If so are you sure they are not 270 degrees out of phase as it will look identical?
This will cause vibration at speed and more keenly when the suspension is extended and those joints are working harder.
BagusJeep lives in Bali.
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE