Okay, if you do not live within easy driving range of Red Bluff, Ca (south of Redding, Ca) file this. In 2005 I got a Granby mounted on my 99 f-150 1/2 ton 4x4 and had all sorts of problems like you have had-springs, shocks, blowing out air bags, tires. reverse dump, bad ride, etc. One day, I was at the local ford dealer and mentioned my problems to one of the mechanics (a recent transfer from over the mountain) and he mentioned and recommended a shop in Red Bluff that specialized upgrading a 1/2 ton 150s into a 3/4 ton capacity for like using a camper. Moore's Blacksmith shop in Red Bluff-I called, set up an appt.. went over the hill and it took about a half a day later with a new set of specialized springs and I was off into the wilds.
I have never had any more problems with ride or anything -okay this was about 15years ago, and I just checked the phone book, and he is still there! The phone number is: 530-527-4501, address is 742 Oak Street.
I have no idea if they still do that work, so it might be worth a call. Next to a nice park that borders a nice river across the street and there is an okay RV park down the street. I have no idea where this line came from and iI cant figure out how t get rid of it-sorry!!!
Smoke
PS: I think the cost back then was around $800.00-could be wrong.