Spreading out the thought pattern:
Since you replaced the swing arms, bushings and all, and grease helps a bit, I'd suggest you check the swing arm shaft nuts are properly torqued/shimmed (not sure if they should be), if you have adjustable camber brackets that they are secure, that the handbrake cable quadrants are properly supported (not too loose), that the brake backing plates are properly secure, that the rear brakes are properly installed with the springs the right way round, that it isn't the exhaust system making contact on a bump...
Since you installed steel coil springs, you probably have adjustable trumpets. If you've set the ride height too low, you may be getting slack in the suspension when the wheel drops. The springs should have at least a little compression from the swing arm with the wheel on full droop while jacked up.
You said you also replaced the shocks. Are they the right length for your suspensions set-up? Are you certain they are properly tightened?
"Hang on a minute lads....I've got a great idea."