Daystar makes poly mounts. But you will experience some vibrations. Exactly how much won't be known until you put it in. I put one on under my auto tranny. Now I had heard that there would be some vibes, but that it is bearable. :bs: The vibes I had were teeth- and window-shattering. No way would I ever get used to that. It came out after one day.
Remi (aka Kejtar) made a good point. If you look at a factory tranny mount, it is angled - one end is higher than the other. Aftermarket mounts are level straight across. That causes the vibes. But some people have better luck with poly mounts.
Like I like to say, "Not all XJs are created equal."