Check the letter codes and then check the actual ratio by putting on a lift and turning the drive shaft and wheels/tires. Also, count the number of bolts on the differential cover, if original, it should be a 10 bolt. Finally, compare what you have to the pictures/references at jdrace.com

As far as purchasing parts, I've gotten both my rearend gears and posi units from Randy's Ring and Pinion. I think they go by the name Randy's Worldwide now. It isn't the cheapest option but I've had no issues getting the right parts.