hello, 88bonnsse there'* a simple procedure for checking for a bad tranny/engine mount.
You need two people, one person in the drivers seat, the other looking in the engine bay looking for abnormal movement of the engine and tranny. Have the person in the drivers seat start the engine and have them put their left foot on the brake firmly. Now have them take their right foot and gently step on the gas pedal momentarily. Do This while shifted in to drive and reverse. This will cause a resistance opposing the engine torque and the engine will want to move. If you have healthy mounts there will be minimal play.
Also when accelerating its normal to feel a slight jerk (a smooth jolt if you will) as the transmission shifts to a lower gear so you can access your engine power. Now if you accelerate in the normal manner and it feels like a big KICK, like someone hit your car with a giant sledge hammer, then it may be a symptom of your transmission not synchronizing things as it should. In a perfect transmission you should feel nothing as you shift gears.
Hope this helps.
1999-2007: ___1988 Bonneville LE LN3 (Vin C) @ 273,000 miles
and still going strong (<-Faded sun fried grayish ie needs paint)
2007-Present: ___1993 Bonneville SE L27 @ 210,000
Speeding around So Cal (<-Beautiful Forest Green)
Mods: Sylvania Silver Star headlamps. Remote start system with pageing device. K&N air filter. Tranny Cooler. High-flow cat with Magnaflow muffler. Full Class II hitch setup.