rhythmic vibration

Okay, the vibration that we felt while the car was on the lift was at a constant speed, no matter what the rpms. While on the road, the vibrations increase with rpms. The faster I go, the less interval time between vibration. The opposite is true, the lower the rpms, the longer the gaps in between vibration.

We thought the vibration was engine related, except when we put in the 294 gears, the vibration disappeared. It first recurred on decelleration at 50 mph and slower. It has gradually worked it's way to being constant, regardless of speed, and regarless of accelleration or decelleration.

Exhaust has been an issue. We ran dual exhaust from Summit. All hangers have rubber insulators. Occasionally, I hear exhaust rattle while idling, but it dissappears once I get moving. Could it be there in the form of this rythymic vibration? We installed Gabriel Air Shocks to lift the rear of the car for more clearance, but the passenger side is still tight going over the axle and passed the gas tank.