I believe every poster here that can hear/feel this is actually feeling a bind/slip in the steering column at the steering u joint.
Very common, not just to Jeep.....just about every manufacturer has had issues with this design.
As most have noted, the feeling is worse with the engine off because there is no assist.....the steering shaft is really being loaded hard with the engine off.
Some of us that have been on the various forums from the XK's beginnings should remember this issue when these things were only a year old.
Many of us had the dealer replace the shaft under warranty, only to find the sound/feel had returned in just a few months.

Anyways, highly unlikely this is a rack problem.

Rob