'71 trailing arm questions

[mister.freeze]

I have a '71 longbed that was made into a trailer. From the get-go, it was bouncy as a trampoline. I put some Monroe Gas Magnum shocks off a Suburban on, thinking the heavier truck had stiffer shocks. They weren't any better. Could have been bad to begin with as the Suburban was just sitting around. My Blazer has a 3/4 ton suspension kit on it, and when I replaced the Pro Comp shocks on it with Bilstien's I warrantied out the Pro Comps and put a new pair on the trailer. Still just a bouncy house on wheels. What gives? The original coil springs are on there, is that the problem? I wouldn't think so. Is it just too light? I wouldn't think that would be the problem either. Thoughts? Thanks in advance.