it's a 68 cutlass, but i figured the same would apply to a chevelle. the rear end sets lower than the front by about +1". you can see it in these pic if you notice the tire heights at the wheelwells. driveway is sloped so the car looks level.
http://groups.msn.com/mtbcarsales/shoebox.msnw?action=ShowPhoto&PhotoID=255
http://groups.msn.com/mtbcarsales/shoebox.msnw?action=ShowPhoto&PhotoID=254
http://groups.msn.com/mtbcarsales/shoebox.msnw?action=ShowPhoto&PhotoID=260
anyway, it has air shocks on the back with a broken line so they are probably all the way down, but still have some movement in them over bumps. the fronts are a little "floaty" and seems to bottom out over big bumps, so i thought about putting a new set of kyb gas shocks all the way around. i looked under the back when i was trying to find codes on the 12-bolt and the springs didn't seem weak or close together. they had no rubber insulators. i don't think just replacing the shocks and adding insulators would bring the rear back level unless i used some of the heavier rubber spacers. we are really on a budget here, so as much as i would like to redo everything, i have to be careful not to sink to much money in the car. know it's had to tell for sure just by the pics, plan to take it to a local spring shop just to ask what they think. any suggestions?

For the cheapest (definitely not the best) fix, why not just get new air lines for the air shocks and pump em up.

For the "Right" Fix- New Rear springs & new Shocks.

My 68 sat like that. Ditch the air shocks and buy a set of rear springs and regular shocks. You won't be sorry. I had 206K on mine when it sold. The 15 inch newer olds wheels makes it look real nice. Mine had the 14 in. with the stock 442 hubcaps. My 67 F-85 had those wheels. IMO the stocker the better.