Concur on the suspension bushings, with an emphasis on the panhard rod. A couple of years ago while returning from our Colorado/Utah trip, with lots of wheeling involved, my 4Runner had a new trick - a slow side to side wobble at higher speed son the highway. Usually initiated by a road heave or bump. Nice and slow, and not increasing in intensity, but certainly unnerving. I had to double and triple check to make sure I wasn't subconsciously doing it via steering wheel input (I was countersteering to try and stop it). But no, it was all the truck's doing. At the next gas stop I wiggled the truck vigorously side to side, trying to spot a loose bushing, didn't see anything. Kept driving it home, and learned to do a sort of long curve (in the lane) to 'reset' it when it got started.
A week or two later after going through the rear end (HD lower control arms with poly bushings, newer upper OEM arms, SS adjustable panhard rod with poly bushings) I discovered that there was a slight bit of wear on the panhard rod bushings, maybe enough for about 1/4 inch of movement when pushed hard? So little it looked like normal bushing deflection to me. I replaced them and... wiggle/wobble/oscillations were all gone.
