Anytime you modify an oddball vehicle (such as the H3), you will have to make your parts...or find some other application to work on it. Most of us agree that Rancho shocks are completely useless...other than they bolt-on without any mods. So what do you do if you want to upgrade the Rancho shocks? Get out the tape measure, determine the appropriate length, determine the appropriate connections, and then take a stab. Most likely you will need to fiddle with the valving to get the ride quality you are looking for. Jeep JK shocks are very close on the valving but, the connections are totally different.
A stock H3 needs front shocks with a shaft bushing at the top, and a super-strange, double-shear bolted connection at the bottom. The rear needs to be a GM bar-pin at the top, and a bushed eyelet on the bottom. This makes them sort of "special" in terms of aftermarket shocks, and it may take some searching to find a bolt-on replacement. Most good quality aftermarket shocks (Fox, King, etc.) will be standardized around the eyelet top and eyelet bottom configuration. This seems to be the most universal way to mount shocks, and I recommend converting to this configuration if you are doing MAJOR mods to the suspension. It makes shock selection easier, and vastly increases your choice of shocks to choose from. But since you are still using "stock" connections with the Rancho...I would try to stay with that configuration.
For the rear, some guys use bar-pin eliminators on the top but, these have their own set of issues and I prefer to keep bar-pins rather than run eliminators. In most cases you can press-out your old bar-pins and re-install them on some new eyelet-to-eyelet type shocks. Other GM trucks share the same bar-pin dimensions, so you might searching for a Silverado 1500 application. Some Ford and Jeep applications also use bar-pins but, the dimensions are slightly different so expect to spend some time with a die-grinder to get them to fit the GM bar-pin brackets on the H3's frame. I would think the valving for a 1500 GM truck would be close enough.
For the front...its anybody's guess. I would suggest you start with searching a Colorado/Canyon application. If you don't have any success, you might want to consider converting to a standard eyelet-to-eyelet layout so you can run any shock you like. I think it would be fairly easy to fabricate some adapters for the top and bottom attachment points. But the big concern is that adapters take away length...and you will loose probably 3" of shock travel to do this. So perhaps using an adapter on the lower control arm and a totally new "hoop" on the top would be a better solution.
