2007 8600 single axle/need more ride height

try adjusting the leveler valve to increase air going to the bags. some have a threaded end others you have to bend the rod. don't take it to far. you don't want to over extend the bags. if you can spend some money. put on a set of road king shocks. there adjustable. expensive.

 

Shocks of any kind will not keep the load from hitting the suspension stops. When ride height is changed it affects drive line angle.

It does sound like there isn't enough air in the bags, it is possible the previous owner/driver was in search of a better ride and adjusted the leveling valve. Just be sure to check the drive line angle after adjusting. I would say the dealer can give the specs on suspension height for that truck

 

You could always do it Border Brother Style with a couple two-by-fours and some Muffler clamps to hold the wood in place

 

I believe that is what I said. adjust leveler valve to increase air to bags. Rear shocks absorbed upward pressure not downward. unless you turn them upside down. which may be a option. but if you increase bag pressure beyond spec the shocks have to be able to handle the extra pressure. that's way I mentioned road king shocks there pressure adjustable.