Rear tray back issue

A friend of mine has had the rear of his Disco tray backed. I've been doing other works on this n he says it drives awful, I've watched it drive on a motorway doing around 50mph. The rear axle looks like it just skips along, he's running 35inch Silverstones has +11inch shocks, +2inch progressive springs, extended Johnny jointed trailing arms. Do you think putting twin shocks on the rear it with smooth the rear axle ride out along with balance beads?
Regards Steve

Hard to say really have you been through all the bushes, ball joints and mountings?

Yer it seems as if the springs are to bouncy, like the shocks aren't dampening it enough. But I'm wondering if it's just the tyres with them been so big.

Tyres got naff all do it with it. Will be the shocks.

Plus there'll be a lot less weight on the rear so the springs might be a bit 'stiff'.

If this link works here's Owl weighing the front and rear of his custom machine to calculate the best spring rate for the job.
https://www.landyzone.co.uk/land-rover/alrc-project.160626/page-33