Vibration -- now you feel it, now you don't

My '98 jeep cherokee has developed a vibration which sometimes is there, and sometimes is not. When the vibration is there, it can typically be felt at all speeds sometimes as low as 35mph, but is most noticable at highway speeds. Any given time I drive the same stretch of highway (same lane, even), the vibration may or may not be there, and may be “mild” of “very bad”, but it will be the same for the entire stretch. To clarify, I can drive 10 miles of highway to my sisters house 3 times in same day – so 6 stretches of highway, 3 there, 3 back. Each entire “stretch” will randomly have smooth, mild, or bad vibration. It seems that stopping/starting will randomly change which vibration (or none) that I get. But on a long highway trip it can change randomly every 10-30 miles.

Most typical causes of vibration, it will always be there, like wheel balance or allignment, right? So what can cause this?

I did have a flat tire repaired a day or two before I first noticed (and I don’t think I drove much those 2 days), and I took the tire back and made them balance it, but that did not fix the problem.

Which leads me to one theory I have yet to check out: could the tire repair have left a small object inside the tire? My thinking is that this object could shift to different places in the tire no stops/starts, and it would tumble at low speeds, but when the tire is spinning fast the centrifugal force and friction keep it stuck in one place. Sometimes the object is close to position where it was when they balanced it, so will be balanced and smooth. Sometimes it will be near the opposite position for the worst vibration, and other times in between for more or less mild vibrations. Does this sound likely at all? (yeah, I know it will be easy to verify that theory when I get around to jacking it up.

What else could cause this?

Oh yeah, f I had to say, I would say I feel the vibration more in my feet/seat rather than in the steering wheel.

Have you had the driveline checked? Universal joints can give you this type of trouble. Listen carefully as you shift the transmission from drive to reverse while holding the Jeep from moving with the brake. If you hear a metallic pinging or heavy clunking, it is definately in the driveline.