I have a 1991 F250 Super Cab with the 351 V8, Superchip, Jacobs Ignition and Gibson 3" Single Exhaust. I have recently begun to experience a vibration that occurs fron 1200 - 1800 RPM only under load. There is no vibration under steady throttle or when I back off. The vibration smoothes out above 1800 RPM or if I kick the transmission down a gear (increasing the RPMs). It dosen't happen in 1st gear, is somewhat faint in 2nd gear( 25 MPH), is strong in 3rd gear (35 MPH) and severe on Over Drive (45 MPH). I have been chasing the engine thinking it was a misfire, but all seems to be fine there. Now I'm thinking it might be something in the drive train that is related to where the engine is in it's torque curve. Anyone have any ideas? Thanks!

------------------
My tow vehicle: 1991 F250 SuperCab, 5.8 V8, SuperChip, Jacobs Ignition, Gibson 3" Single Exhaust.