Couple thoughts.
First, shocks dont lift the vehicle, they dampen movement of the axle. There is an execption to this in air or gas charged shocks... but they really dont work for "leveling the vehicle". I helped a guy weld up his frame after he put big old monster shocks on his truck, the mount on the frame the shock bolts to cracked the frame as it wasnt heavy enough to support it.
To level a vehicle, you have three choices. Add another leaf spring to your rear leaf springs, get air bags, these mount between the axle and the frame and will indeed lift the vehicle. Some luxury cars use these with a compressor to "level" the car when people sit in the back while maintaining a smooth ride. Primarily though, you see the air bags on big heavy dump trucks or other industrial sized equiptment like Semi's etc etc. Bags work for loads in just the truck bed (atvs, camper etc), or from hitch weight when hauling a front heavy trailer. The final choice if your unevenness is due to a trailer is an equalizer hitch.
The disadvantage to bags is unless you buy a really high end automatic system, you will be adding / releasing air manually.
A quick link to show what I'm talking about when I refer to "bags":
http://www.autoanything.com/suspension- ... MgodJg4ALAOf the above, it sounds to me your truck just needs an equalizer hitch for when hauling a trailer. Lots of options with those but I got a basic model at the local State Trailer Supply. It basically lets "arms" from the hitch connect to the trailer and will "lift" the rear of the truck and apply the weight to the back of the trailer. Sounds crazy but it sure does work. I cant remember what I paid for mine, and statetrailer supply's website is currently down. It looks like this:
http://www.thehitchstore.com/trunnion-s ... -1325.html-DallanC