Technical Cruising at 65-70mph help

I have to grin also but I will add something.
Driveline and 2 speed rear ends like the Mitchell, the Ryan, several foreign types and the Columbia add a LOT of UNSPRUNG weight which is exactly where you do NOT want it. Whether you believe in the laws of physics or not, they affect the handling of the car adversely. I have removed several customer installed of the above and installed transmission type overdrives because they didn't like the ride and handling characteristics.