My 87 Pontiac had overload shocks, or the shocks that have the coil spring around the outside, and they are advertised for increasing a vehicles load capacity, but the problem I had is that every time I hit a bump they would make easily noticeable odd noises.
So I replaced them with a pair of adjustable air shocks.
Monroe MA717 are the air shocks I got and put on the rear of my Pontiac, a bit of warning though with any air shock, there is a good chance that they will gradually leak down so they would need to be checked even just for normal driving, I usually check on my Pontiac at least once a month but I don't load that one up
On my Buick that one is a different story as she came with a self leveling system, but I had to replace those shocks. ACdelco 504110 are what I bought and installed.
with both I had to route air lines, for the Pontiac I went through the frame to the rear and along the bumper and used one of the license pate bolt holes to hold the inflator for the shocks.
For the Buick I just had to route new line from the shocks to the main line from the compressor and put a T junction on that I got from Grainger. I had this one loaded up with soda and food for a concession stand and the compressor was kicking in about every half hour for a couple minutes to bring the car back up to level, but I think the Grainger push fit T I put on wasn't sealing aswell as it should, but the rear springs are good enough where if I have no load she sits level.
If it's anything like my two, if your shocks are already getting close to needing to be replaced it would be best to do them aswell as the springs, as for me it was easiest removing the springs and then the top mounting bolts for the shocks as with the spring out you have a lot more space to finagle your hand into the small gap between the body and the frame.