JP,
I have the same spring/shock set up as on your van and have no handling problems at all. I have the CV900 wheels w/BFG Radial TA KO's (225/70-16) on the ground. The Gary Lee tire and ladder on the back.
 
My OPINION is that your van has some sort of alignment problem from the sounds/descriptions posted already and someone already suggested rear wheel alignment, but that would be my first suggestion to get thoroughly checked out, both front/rear.
 
Bob Stevens
'87 Syncro Westy
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