As fiedlerh has suggested you may want to check your driveline angles. I had a GM A-body (Olds Cutlass) that developed a severe vibration at 55-60 MPH (~2500 RPM) that was clearly a driveline problem. At 2500 RPM in 2nd gear (T-350) no vibration at 60 MPH with gear in neutral same vibration. This all occured after a Engine/Trans rebuild. The motor mounts and trans mounts were replaced. The issue was the Energy Suspension urethane trans mount. This mount is way to tall for may applications ( I believe height is ~ 1.927 inches). I purchased three different rubber mounts (Anchor, NAPA and Pioneer) all had different heights the Anchor 1.88, NAP also about 1.86 and the Pioneer 1.78. I replaced the Energy Suspension mount with the shorter Pioneer mount and the vibration was gone. As indication on how far off the Energy Suspension mount is, the GM (OEM) trans mount for my 73 Firebird with a Muncie is only 1.80 inches even though ES claims their mount fits.
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