Vibration In Rear

I Have A 01' Exp. And After It's Been Driven Awhile , And It Shifts In The Last Gear It Makes A Fast Vibration But It Only Does This In The Last Gear, Could This Be My Driveshaft Missing A Balancer Or Any Advice .

To me it sounds like torque converter shudder. You can purchase a product at an auto parts store that pours in the transmission fluid and cures the shudder. i have talked to a lot of transmission rebuilders and they swear by it. I believe it is manufactured under the name Lube Guard.

Change transmission fluid in transmission and torque converter... that'll fix it. 14qts?????