There are tons of great discussions on the Sprinter forum about improving the ride quality and handling for Sprinters.
https://sprinter-source.com/forum/index.php?
I think you could summarize most of this by the following order of Sprinter suspension upgrades (in order of preference and magnitude of effect)
1. Better tires and correct tire pressure.
2. Better rear shocks (this is a biggie, the stock shocks are lousy) - the custom tuned Fox shocks from Agile Off Road and Van Compass would be the best followed by Koni's
3. Better front struts - Koni's probably best currently available
4. Rear leaf spring upgrades - like the leaf spring assist products like the ones from Upscale Auto and Sumo Springs
5. Bigger rear sway bar - improved handling but worse ride quality
6. Sumo Springs
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