You said you can push the car while its in park? That's the diff. If the trans. was out, park would still engage, its a pin that locks the trans., and keeps it from moving. Check the 4 sale/wanted section, and get a diff with whatever gearing you think would best fit your needs. Iirc, stock on all n/a's is 4.30:1, but you could get better mpg with a 3.93 out of a turbo. Its possible you could get a new ring and pinion, but I would HIGHLY recommend you get a professional shop to install it for you. The gear mesh has to be perfect, or you'll tear it up again.
If you want to be sure which one it is, put the rear on stands, and try to turn the driveshaft with the trans. in park(it shouldn't move), then try the wheels. If the wheels move, but not the driveshaft, the gears in the diff. are stripped.