Been there, done that. After you assure that you don't have anything really loose with your front suspension, you can screw around chasing minimally worn components for thousands of dollars if you want . . . like I first did. But, if your van is like mine, it won't fix it. Rather, it is a luck of the draw problem with all coil over solid axle suspensions incl. stock Fords and Dodges. (Chevy's are indep. frnt suspension.)
But an opposing dual gas pressurized shock system will do it. Just read what TurboStew says and do it for some piece of mind:
Quadvan Death Wobble:
Mine is a Salem Kroger 4x4 as well. Tell me how you lock your hubs. I may have another suggestion.