This is from a post I put on another thread in April. It may have some relevance to your ride qualities now.
Posted - Apr 15 2004 : 11:09:37 AM
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"A short wheel base will not ride quite as good as a long one. Also, if it's 4 X 4, the heavier front suspension is going to give a harder ride. And on a concrete road you're dealing with harmonics; Trailer wheels hitting a hump about the same time as the truck is settling down from one. Just a passing thought. This is something I experienced over the years of truck driving as semi-trailers went from 35' to 48' and 53'. The bridge between the drive axles on the tractor and the trailer axles matched the joints in the pavement and in some areas, it would almost yank the seat out from under me. When we went from one trailer to twins and triples, it all smoothed out.
One thing I might add; Quite often if one goes 5 to 7 MPH above or below the speed that is giving you the harmonic slap it will smooth out."
Also if the ride is too intolerable, National Seating makes air suspension seats for large pick-ups that have an isolator on them. Might check them out. Just a thought.
This link will take you there;
http://www.buytruckstuff.com/product...=16&p_gpID=168