my outback starts shaking very badly when doing 60-70 the floor shakes to what is this, off differentials? or alignment.

 

It may just be unbalanced, or bent wheels. If they are alloy rims, they may even be cracked. I had it happen to my WRX more than once (I drive a lot of back roads with rednecks in big trucks taking up the whole road).

 

check wheels, air pressure, balance, and alignment...

 

I'll almost guarantee you that it's off balance wheels. The best thing you can do to get your wheels as smooth as possible, is to find a shop that has a Hunter GSP 9700 "Road Force Balancer", and a set of Haweka adapters (they are lug centric adapters that mount the wheel to the balancer through the lug holes, just like they would be mounted to the car). I left a link at the bottom to Hunter's web page. Click the link that says "Find a GSP 9700 near you!" Once you have a list of shops that have that machine, call and ask the shop if they have the Haweka adapters.


http://www.gsp9700.com/