Perhaps exhaust resonance....
I heard stories of the sn-95's having rattling/vibration using certain h-pipes so it's possible that you might have a similar issue. If it's a jarring, it might not be but you mentioned that there is no vibration in neutral and this is when there is no load and less pulse pressure to cause vibration.
Maybe I'm just rambling =P
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91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior
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