Those shocks were common on old school lifted 4x4s. I imagine Ebay could produce some, or I believe there are kits to convert Rancho 5000 series shocks over.
I bet those Rancheros would be really stiff shocks for what he has there (not to mention pricey). I put a set on a Nissan 4x4 I had once and the ride turned into the equivalent of a log wagon.
I'm not sure what the coilover shocks add to that suspension anyway, though...Having both the leaf spring and coil over shocks is nothing but redundancy...leaf springs provide a bit of frequency absorption through friction between leafs and don't typically use shocks (on trailers anyway). However, in that application, I presume those leaf springs only serve the purpose of a fourth link in that suspension system and keep the axle in place while the coil over shocks do most of the suspension work.
With that in mind, I would consider something like the Gabriel load carrier shocks. They have some with a variable rate spring that only starts to add relative load carrying capacity under some compression...that should be pretty good for that application. You would need to call someone and give them travel dimensions for that shock to figure out which version would fit.
https://www.summitracing.com/search...ing&keyword=gabriel%20load%20carrier
