In my '91 4.0 I have a shimmy that I have been unable to track down. The jeep rides smoothly except for a rattle/shimmy when I hit a bump (pavement patch, expansion joint, etc). It also does it randomly on the highway between 60-65mph where it does not seem to be related to rough pavement. It feels like it is coming from right below the drivers seat. I replaced or tightened the exhaust system and tightened everything I can think of. Any ideas? Also, I do not feel it in the steering wheel or shifter.