Have you inspected your wheel bearings?
A long shot as I wouldn't expect failure here to cause vibration as much as outright noise but it sounds like you've already addressed the most likely issues.
Also take a look at your drive shafts (although you probably already have) - maybe they are out of balance some how?
I'm not 100% convinced that the ambient temperature isn't just a red herring.
Tough problem - hope you find it though - and, of course, let us know! Its rare around here for someone to have a completely unique problem.