Originally Posted by VehiGAZ
No problem - glad you got it sorted out.
There are few symptoms in a vehicle that are almost always one or two possible problems, but a vibration in certain speed ranges is one of them.
Badguy... do you by any chance have Tone's seat rail extenders installed? I put them in mine, and they didn't fit so well, and they eventually stressed the seat frame rails until a weld cracked. The passenger seat had a nasty shake to it at that point, but I had the frame rails replaced (under my extended warranty -suckers!).