I've got a 97 SH with 40,000 miles that had developed a weird vibration. I’ve searched the archives for a while and could not find anything similar. Here’s the deal:

There is an entrance ramp to the freeway that I love to have fun on. It is a 90 degree turn, followed by a straight, then another gradual turn onto the freeway. I usually keep it in 2nd and floor it out of the turn and into the straight. Then I let off the gas and lightly apply the brakes to slow down to prepare for the next turn. That’s when I noticed something weird. The whole car started vibrating/shaking. It happened around 6200 RPM until 6100 or so. Then again from around 5500-5400. I thought it might be my brakes, or the road surface or something, so I tried it in first in a smoothly paved, empty parking lot, while going straight. I put it in first, brought it up to around 7000 RPM, and let off the gas. It made the same crazy vibrating/shaking. The car still pulls fine, and sounds fine. No funky idle or anything. It seems something is off when I try to engine brake/decelerate from high RPMs. A friend suggested I check the motor mounts by putting the car in gear, making sure the parking brake is off, and then rock the car back and forth with the hood open. He said the engine would shift around the engine bay irregularly. When I tried, the H22a4 just sat there, maybe moving a half an inch. I also check all my fluids regularly. Everything is clean and full. My only mods are: Iceman intake, DC Headers, 40 shot NOS dry kit, NGK plugs, and Toyo tires.

Thanks in advance. Any help is greatly appreciated!!