Looking at them, I am surprised you say they made your ride smoother. I would have thought the opposite would be true.
I am considering
Sulastic Shackles for the rear axle to soften our ride. Model SA-06HDD
HERE works with all year cut-away Ford E350 & E450 chassis. But at near $400 to find out if they are effective, it has me a bit gun shy. I also wonder if they increase, maintain, or decrease the rear end height. They do seem to offer a logical and simple approach in softening the ride. I'll have to keep an eye open for a better price, making it worth the risk of disappointing results.
Sulastic Shackle SA-06HDD

I found this picture comparing the Sulastic to the stock shackle for another vehicle. Sulastic offers a cushy hinge action.
Concerning is the distance between the mounts being less on Sulastic than the stock resulting in a lower rear end.
And there is no weight yet on the Sulastic.
