Rear of car all over Road when it's raining

Worn out tires?

Inspect the rear suspension for rust damage or wear and tear.

Stick or auto? If the former, you may have a worn out viscous link; evidence in the dry is crabbing when cornering or parking; in the wet you'll slip the rear around. If auto or stick, look initially at tires for both tread depth but especially scalloping or running backwards if directional! Rear suspension issues, like broken off swaybar links or worn rear bushings or struts should provoke sloppiness both in the dry and wet. If auto you may have worn AWD diff internal bearing seats. You'll notice this as excess drag when cornering or parking...similar to the manual issue above. Try changing the ATF and adding a tube of Trans-X Posi-Trac to the ATF. This is a tried-and-true trick to lubing these worn out Teflon seats. Good luck, and keep in touch. Ern TSG/B