Free shipping based on multiple price ranges?

Hello,
we're offering a discount program similar to the following:

Purchase over $100, get 5% off.
Purchase over $250, get 5% off and free ground shipping.
Purchase over $500, get 10% off.
Purchase over $1000, get 10% off and free ground shipping.

I have the percentage discounts set up just fine, but is there any way to set up multiple price ranges at which the free shipping option kicks in?

Is there a specific place I can try and write a function that will look at the final shopping cart price and depending on what range it falls into, enable the free ground shipping option?

Or is there possibly a module that I just haven't found?

All the best, you guys are aces.

You could customize the Flat Rate flat shipping module to display as Free Shipping Ground and set the charge to 0.00 ...

Then, customize it to only show when:
(total >= 250 and total <= 500) or total >= 1000