I'd bring it to the springshop, if the frame isn't bent. Sounds like a spring problem though. Prob. lookin' at around $500 would be my guess, but things are a little different here in Maine from what I understand. Good luck with her....must be a nice plow rig!!
Another possibility, if the springs aren't the culprit, could be the body mounts themselves. If the "rusties" are taking over, the formed sheet metal that the body cushion bolts to may be collapsing. Or, the rubber cushions themselves could be getting tired.
Check the springs out too though, it may also end up being a combination of all three factors.
I'm with Rob on this one. Check the body mounts where the floor steps up in the rear behind the driver's seat. That one usually rusts away fast.
I would also look for a cracked leaf on the driver's side rear.
The Blazer is tricky in a situation like this. Since the body is "one piece" from the cab to the tailgate.
Cab sag on pickups is common. Sag due to body mounts on a Blazer is a little less common.
Even if you look at things in broad daylight, use a drop light or bright flashlight to really get a good look when you crawl around under there. I like to use a wooden ball pein hammer handle to press on rusted parts to see how bad they are. Watch your eyes, rust really hurts. I still have a piece lodged in my eye from 8 years ago. Anything that the hammer handle presses in, is no loss, as it was no help structurally speaking. Then you can assess exactly what you need to make the repairs.