Replaced rear bushings but still 'unstable'

[Tim240Z]

On my C20 Burb, I replaced the track bar and control arm bushings in the rear with urethane ones, but at speed the rear of the truck still seems very 'loose'...
Hard to put my finger on it, but it feels like the rear end is floating around back there.....never felt like this with my 71 truck....maybe just the way these burbs are? Very unsettling feeling especially on the freeway, going around a bend with a rough, or washboard type road surface....takes every bit of concentration to keep it in a lane.
I have replaced the steering box and the pitman arm in the front and all else appears tight and good....

Any suggestions? All shocks appear and feel to be good (paint still clean and the rears had lots of push to them when I did the rear bushings - unbolted the bottom mount - they were obviously done fairly recently by the PO)
Tim