Intermittent Vibration at Highway Speeds

I have a 1995 GMC SLT Jimmy 4X4 that experiences a vibration around 60 to 80 MPH....the problem is not constant. It seems to begin...gradually becomes worse then goes away then quickly returns. The whole truck vibrates but it seems worse below the driver's seat and in the rear of the vehicle...It does not feel like it's coming from the front end....All the mechanics I talk to seem to believe it's in the tires but after having new ones installed it seems to be the same reguardless of the orientation of the tires. I've had the rear end rebuilt, the drive shaft balanced, the U-joints replaced, the idler arm replaced, new motor mounts installed, the transmission mount looks good and new half shafts with CV joints have been installed ....I'm quickly running out of money and patience to troubleshoot this any further....Any ideas ???? It does have the stock sized tires mounted on slightly oversized rims if that helps any but I don't think this would cause the intermittent problem I'm experiencing.....
SEDIVER

If the mechanics you are dealing with are telling you "that they seem to believe it's in the tires" then that is a good indication that they have not properly diagnosed your vehicle and are just throwing parts at it hoping that one of those parts fixes the problem.
You need to find a technician that has an EVA (electronic vibration analizer).
This is an electronic tool that senses vibrations and records their strength and frequency..with a few calculations the technician can determine what rotating componant or its related group is the cause of your vibration.
Vibrations occur from the following situations
Out of Balance
Out of Round
Damage
Excessive runout
Operational characteristics
componants contacting each other
improper working angles or phasing

The thing to remember is that there are componants that rotate the same speed as tires that may be the cause of your concern.

Following up on my original post years later:
I recently, I bought a new set of tires for my 1995 GMC Jimmy SLT and thought I'd try a little experiment. Along with the tires I bought an OEM set of Chevy 4X4 alum. alloy wheels (I like the look of the Chevy OEM's better than the OEM GMC's). One of the things I noticed right away was how little of an offset the Chevy 4X4 OEM wheels had compared to the "Prime" alum. wheels that came on the truck when I purchased it in 1997. I would say that the "Prime" wheels where offset nearly 1-1/2". I had an alignment done after the installation of the new wheels and tires and took it out on the highway. It had a smooth ride through all speed ranges.