This thread may have alternative Moog parts compatible with your van. Please note, tapatalk posted whole note, I meant to share link. Nonetheless, part links below.
Threads found on caster adjustment on this forum
This one talks about getting shofter arms
This one talks about getting caster really high (6-7). Very informative read, although the poster has high lift (6")
Also, this is a very long but worthwhile read (on another forum). The thread is mostly about steering stabilizer (adapted off an E450), but it talks about increasing caster to 5% for maximum comfort (e.g. post #508 and beyond)
post #517: Alignment bushings that might be needed to help with increasing caster: NAPA 158986 or Moog 8986 or Ingalls 594s
post #525: caster does not affect tire wear. Only camber and toe
Having too little positive caster will manifest itself as "loose steering" and "wandering" at highway speeds.
Ford ships the E series with +3 to +4 degrees of caster. You want to add 2 degrees to have +5 to +6 degrees. Google "E350 Handling" or "E350 Caster" for lots of info.
Estimating caster w/o going to the shop
At this point I am somewhat "sold" on the caster idea, provided all other parts are in good shape.
Couldn't find on Ingalls, but found recommended Moog K8986 parts on amazon. Ordered them and will bring them to alignment shop. Can always return if not needed.