Drivetrain Vibration 2003 E500 62,000 miles

[justncase]

I have a strange vibration that only appears when climbing (the transmission is in D and has not down shifted). It feels like a straight shift in 5th gear trying to climb a steep hill. The car begins to shake or vibrate (not violently). I just replaced the tires and alignment (this was going on beefore). Finally went to the Dealer this past week and they replaced the transmission mounts they also checked fluid level. They said there was no burnt smell or metal in the fluid. It is still vibrating. The service manager says that it might be a rear axle. It seems to have gotten worse over the past 3 months. When the vibration starts I shift from D to 4th (it still vibrates) then to 3rd and it goes away.