Believe it or not - check your motor mounts!  And I mean +really+ check 
them, not just "yeah, they're there, so what?"  Plus, it's really fun to 
do in a Vanagon; pop the engine lid & have someone drive around for you 
while you're on your stomach watching the road rush by - cheap thrills - 
anyway have him/her feather the throttle on-off & it will soon be obvious 
which mount is bad: & remember, one having gone probably ruined the 
others, so.......