"extended rebound" shocks
 

http://www.google.ca/url?sa=t&rct=j&...VGN1cA&cad=rja

The shocks that were on the trailer when I bought it were Hendrickson S-23651. I'm not 100% sure these are the right shocks, but they're described as extended rebound shocks. What the hell is that? I know rebound is when you compress the shock and it extends out naturally. The trailer rides like crap I'm also considering the S-24579, which is a high damping model.

Shock absorbers for big trucks and class 8 trucks by RoadKing

They are VERY expensive, but I put nearly a million miles on the set on my tractor before one of them (left steer to be exact) started leaking. Also, they allegedly offer six or seven times the damping force over stock shocks and are rebuildable for a reasonable fee. When it’s time to replace the shocks on my new Reitnouer, I might actually put them on the trailer.

There’s another thread on here somewhere that discusses shocks. You might want to read that one as well.

I don't think the problem is the shocks, but I ordered new heavy duty ones anyways. Turns out the drums rode like crap, much better now.

"Extended rebound" is probably another way to say that the "out" stroke is stiffer than the compression stroke. This would make the shock what is known in the racing world as a split-valve shock. Not sure what this does for you in a trucking application.