Question...is it a "bounce" - vertical...or a "wobble"- lateral..??
There are several issues which affect the rear end.
1. wheel balancing - check
2. brakes binding / raise car wheel and turn- check for any damage to true spin - ie no damage to rims and no brake contact to discs
3. warped rear brake disc - as above for # 3.
4. inner CV joints on drive shafts are dry - remove large rubber boot clip on driveshafts, 10 pumps of CV grease and reclip with stout cable ties
5. loose rear stability bar - raise car and check fixing points on bar to body.
6. broken rear spring - raise car and do careful visual check.
7. propshaft rear universal joint worn or seizing up - remove propshaft and have it checked - also for correct balance.
8. propshaft center bearing bushing has collapsed or is weak - raise car and jiggle propshaft adjacent to bearing and observe for excessive movement
9. rear shock absorber collapsed - remove shocks and test
10.Differential mounting damaged / torn or bootwelds to cross member have separated. Raise car check all diff mouning bolts an also diff mount bracket. Remove boot carpet and look for tell tale boot weld separation on left inner area above diff mount.
11.Diff carrier bolts loose - which will affect the trailing arms - raise car and check the integrity of the carrier bolts where they fit to the outer extremities.
12.rear trailing arm bushes - Raise car remove wheel, place jack under hub / raise & lower jack slightly while obseving bushed joints for inapropriate movement at the bushes.
13. excessive cellulite...lipo-suction needed.....
funny thing is a lot of people pull up next to you in there run of the mill cars and start sniggering sometimes whats that all about????? maybe because a blokes driving a yellow car mind you the wives tend to give you a different kind of smile wink wink 
