Good advice given here. Trac bars worked wonders for the rear tail wag on my W22, and the Sumos really helped soften the blow of bumps and potholes.
I did both front and rear on mine so can't say which was more effective, though I think with the Sumo Springs the front is most noticeable when you hit a big bump because you're sitting right over the front suspension.
I think the Timbren springs might be a bit too stiff based on some of the reviews I've read. Might be good for trucks carrying heavy loads, but I think for the cushioning effect you're after the Sumos are a better choice.
I'm also using the P32 Sumos on the rear of my W22, same as YeloDuster.
2004 National Dolphin LX 6320, W-22, 8.1, Allison 1000, Front/rear Trac Bars, Hellwig Rear Anti-sway Bar, Sumo Springs, Roadmaster Reflex Steering Stabilizer, 2005 PT Cruiser Toad