Rear vibration, pass side

[roadierfl]

Hello Group,

I'm getting a vibration from the rear pass side - shocks are good, springs *seem* fine, but when I stomp on the gas it vibrates back there.

I put the car up a lift and pulled the wheel to check the bearings and there was no play there at all.

The axl shaft boots are in v. good shape, and there's no klunking when I turn them by hand.

Also, when it's wet outside, there's a noise that starts back there like a "thump, thump, thump" which comes from the same place, but it goes away when it's dry out.

Any ideas?