All GTs came with an 8.8inch Rear end with an LSD differential and 28 spline axles. Unless the PO upgraded then that is what is your car.
Hard to tell what would/could be wrong without being there. If your axle did indeed break or snap, it may not JUST be the axle...you might need a new differential (probably screwed up the splines on the diff), you will definitely need new axle bearings on the side where the axle broke. It's time to jack the car up, look everything over (all suspension parts, bushings, etc) to ensure all is well, and then probably remove the diff cover to inspect the rear end.
Sounds like your in for quite the repair but no real way to tell unless you start looking at it.
Matthew 26:52-54 / Go Pack Go!
1987 Mustang GT Hatchback - 2V Mod Swap In The Works
1997 Lincoln Town Car, PI Intake/Cams, Sofa-On-Wheels, "Fat Tony" we call it
2001 GT Convertible - D1SC / Geared / Cammed / 440wh 400wt - TOTALED 2/2019