98, I6, auto, 4wd. Ive posted a few times about sputtering and everyone helped me figure that out (thank you!)and now theres an ever so slight shimmy at around 60mph, you can see the passenger seat shake but i dont feel it. I'm figuring my xj just needs to stay right muddy and she''ll keep going strong!

 

Could be as simple as mud in the rims, needing balancing, or tire rotation. Could also be suspension parts going out such as ball joints, assorted bushings, etc.

 

Mine does that too. She needs an alignment that I keep putting off until I can get new wheels. One of the wheels has feathered and makes a "whomp whomp whomp" noise from 40-55 mph because of the alignment being off.