I drive a 1999 mostly stock XJ with a 2 inch lift. I have never had any issues with drive train vibration until greased my slip yoke. It was a quick easy job and I didn't have any issues. Everything seems to be put back exactly as it was before the job. However, I have pretty noticable vibration on acceleration now that immediately goes away once I let off the gas. What could be the problem here?

Are you talking about the slip yoke in the trans ? and did you make sure the the rear u joint is seated .

perhaps a more thorough description of what all you did? Are we talking you literally just unbolted the straps at the axle's yoke then pulled the slip shaft free from the t-case and greased/lubricated then re-installed?

Making sure 1: both the ujoint tabs are still present and good on the yoke, 2: like mentioned above, the ujoint caps are properly seated in the yoke itself as well and the strap bolts are tight.

SOLVED: I had a bad U-joint.

The U-joint between the rear differential and the driveshaft was bad. I didn't initially think that was the issue because it didn't have any give after reassembling everything after the slip yoke lubrication was done.

However, today when I gave the rear driveshaft a shake there was a ton of movement. I replaced the loose U-joint and the vibration I was having while driving completely disappeared.

I love a happy ending.