axle upgrade?

strange engineering is the way to go. they have many types depending on budget and hp/tq figures. here's a link to the online catalog for axles: http://www.strangeengineering.net/catalog/46.html
keep paging forward to see the remaining 8.8 parts they have available. if they don't have exactly what you're looking for, call them up, they are VERY easy to deal with and extremely helpfull. i purchased my shocks and struts through them. good luck!
 
31 spline is an aftermarket thing. You will have to also upgrade the carrier to be compatable. A reinforced rear cover might be a good idea while your beefing up ther rear.
 
I think F150's and similar vehicles have 31 spline axles, so the diff's are available stock, but Mustang 31 spline axles are aftermarket.

I got an 8.8 with at traction lock and 273's, for my Mustang. I wanted to put the t/l in my F150 but realized that it won't fit the axles.