I've heard pretty good reports (on other sorts of smallish vehicles, at least) for monroe air shock systems. You might want to see if they exist for your minivan. The advantage there is that you can adjust the air pressure in the shock to suit the load on the vehicle; it adds some force to the springs.
If you do get air shocks, make sure you get a kit with separate lines and valves for each side. Some of the kits come with a single line for both shocks and a tee for the fill valve (which is just a tire valve). With that setup, the air can flow between the two sides and make the vehicle less stable in lateral rocking, which of course is what you're trying to avoid.
Another thought: do you have any water tanks in your conversion? Is it possible that water is sloshing around and throwing the balance off while doing so?