'Seat of the pants' sensations are hard to diagnose. I agree that body mounts are unlikely to loosen up on their own so unless somebody monkey'd with them, they are an unlikely suspect. The bolts holding the seats to the floor are in kinda the same boat. Unless somebody monkey'd with them, they are unlikely suspects too. I wouldn't totally rule out either & both are easy to check so go for it.
More likely would be the tranny mounts. Several members have posted that theirs have worked loose & in some cases were actually missing bolts.
Also look for the thread where Ldub discussed his waggly rear end. He had found that some of the holes in the frame for the rear suspension had egged out allowing for some squirrely movements.
http://www.vehicross.info/forums/sho...ghlight=waggly