What you're experiencing is something us Previa owners are aware of. The issue is either the coils and/ or the actual mounts are bad.
I like to direct all Previa owners to Toyotavanpeople.com Previa forum:
http://www.toyotavans.net/forum/index.php
or Toyota Nation.com's Previa forum
http://www.toyotanation.com/forum/forumdisplay.php?f=85
There's a lot to be found on this and any other Previa topic on these two websites. Many people have found ways to fix the issue. Also, consider Yahoo's Previa forum as well. I haven't visited there in a while but there is a plethora of info there too. I'm positive that the issue you are having isn't your motor mounts, but check out those websites and see what you'll find.