My original rubber trans mount was so bad the auto shift lever would jump during shifting, and the trans tail shaft could be seen bouncing up and down during idle. Since I was going to replace the mount, I went with a polyurethane type.
I didn't change the front motor mounts. I'm getting a noticeable vibration now, especially during acceleration. Is more vibration to be expected with the poly mount? The only other thing I changed at the same time, was the crankshaft pulley. I am installing the Vintage Air A/C system and the system comes with a heavier crank pulley. It appears to be running fairly true, though not perfectly so.