Don't take this wrong...but you trying to lay all the blame on the salesman/dealership while while not having any culpability in this issue is misguided.
You should have done all the research PRIOR to purchasing this truck to ensure that it would meet your needs and to accommodate the needs of a smooth ride for your back. Pointing fingers at whose to blame now is pointless and a waste of time.
Lower the tire pressure back to the PSI that it was when you took it for a test drive and thought the ride was acceptable and get a good back brace.
