virbation at highway speed

Hi all I have a 2002 Rodeo 4X4 and it has an annoying vibration from about 50 mph and above. I have had the tires balanced a couple of times with no change. I can feel the vib in the seat and in the steering wheel, the vib seems to change frequency and sometimes there seems to be a rumble assioated with it. If the truck sits for a couple of days it will be fine for @ half hour then starts shaking again. Have not been to the dealer yet but will go soon. Anyone have any similar probs with a fix??

If you have the aluminum wheels, these can be a PITA to get balanced properly. IIRC- a "hunter 5000" balancing machine does the job getting them balanced (puts a rolling load on the tire during the balancing process). Only other things I could think of is a worn tire or overinflated tire. Door jamb sticker says 26psig all the way around.
G/luck
Joel