I have a 2000 ss camaro with 4l60e trans... just had it rebuilt less than 500 miles on it. It used to vibrate a little bit before at higher speeds ( 75 an up ). its doing it again but only after 70 an gets alittle worse the more speed i pick up. i dont know if its the trans or possibly the driveline. at like 70-75 when i drop a gear to pass or go faster the vibration is definately there an its getting really annoying and irritating trying to figure it out so please if someone can help id grately appreciate it

 

There have been some posts about rebuilds with "The Beast" sunshell vibrating; however it will vibrate primarily in 3rd gear and not at all in 4th because it is then not spinning.
Other than that, it is usually an unbalanced torque converter or driveshaft which causes vibration.