I just set this up on a test site. Shipping by Order Total.
Seperate Shipping Methods:
Ground Method - $7.95, $10.95, $0.00 (Greater than $75)
3 Day Method - $10.95, $14.95, $19.95 (Greater than $75)
That displays two options, when greater than $75, ground method is free.
Another solution is to click the Marketing tab.
Select the "Shipping" promo and apply free shipping to a particular method. This has a drawback though as it isn't isolated to products.
If this is the scenario you're staring at, I think you have two options
1. Customize the Shipping promo to allow for selection of products in addition to the shipping method to be free
2. Check out Andy Miller's
Shipping Rate Rule Provider this is one powerful tool and I use it for a few clients. It is awesome to say the least.