I just bought a 1973 Gran Torino. These are the tag numbers off the rear end.
WEB-AD5 3CA
2 75 844A

All I can tell from this info is what the gear ratio is, it is a 9" and is not a posi lock. Can anyone help with more decoding?

Also, can this be changed to posi lock without changing out the whole rear end?

 

I would say that you have a 2.75:1 gear ratio. You may even already have a Trac-Lock rear end as the 2.75:1 was availalble as a Trac-Lock. Check the axle code on the data plate. Axle code "2" is an open diff and a "K" is a Trac-Lock diff.