Vibration While driving
 

I work in a garage in Las Vegas and have access to unlimited resources to find and fix the issues I come across. I have a 2000 Ford Mustang 3.8L and have been experiencing a vibration mainly when the transmission is in third gear and when it shifts to fourth it stops vibrating. I had recently installed a rear end assembly with only 6K miles on it and was pretty much brand new except for the oil seals which I replaced. tires are balanced, all the mounts have been replaced. this is starting to drive me nuts lol.

all and any suggestions will help.
thank you all.