Vibration under power

[mikebikeme]

I've got 2006 Outback wagon, 193,000 miles. got 5,000 miles on new tires. I'm getting a powertrain vibration that starts after about 15 min of freeway driving. It will stop after letting off of the throttle but pick back up when throttling on again. I'm thinking its the rear drive line. Haven't gotten under yet to shake it down. I do know the drive axels are solid. Anything I should check when I do go under??? I'm not finding anything on rear drivelines going bad.
Thanks Mike