The Rancho shocks are the same as the Calvert rear shocks. I think Calvert racing bought the rights to them a few years back. Yes, the full Caltrac setup will run you about $1000 (links, mono leafs, shocks), but just the link is "only" about 340 bucks. Honestly though, it sounds to me like you have some really old weak springs back there...I would get the Caltrac mono leafs too if it was me, rather than trying to band-aid the issue....JMO.
Some sticky tires would be good too.