I have a 2000 SSEI...My question is whether or not a REAR trans mount (firewall area) could cause a vibration in the steering and passenger seat or cabin while driving down the road? I have replaced the front trans mount near radiator as well as had all four new Uniroyal Tiger Paw Touring tires road force balanced.
Seems also to be a little bit of "sway" feeling from the engine area when accelerating or variating speeds of throttle.
With a trust worthy friend'* foot firmly on the brake, transmission in drive, pop the hood and have your friend rev the engine up and down a little. Not enough to move the car. Shift between reverse and drive a couple of times. See if the engine is jumping at all.
Like Justin said, problem is probably not a mount, but you can determine if the mounts are okay or not.