[HomerJ]

I had a very similar problem before I tore my truck down for restification. Mine was RPM-related, as it seems yours is (since changing gear ratios affected the speed that you experience the vibration).

I found 2 broken exhaust valve springs - one on each cylinder head. I don't have my truck back together yet, so I'm not absolutely positive that the springs caused the vibration, but that's a likely candidate to me.

If you have a tach, see if you feel the vibration at the same RPM while in park/neutral.

HomerJ