Rear Axle Codes for 05 Mustangs on the door sticker...
BG — 3.31 — non-limited slip, base vehicle with manual or automatic transmission
CD — 3.55 — limited slip, GT with manual transmission
CG — 3.31 — limited slip, GT with automatic transmission
There is also a sticker on the diff cover with the factory ratio. In my picture below, the sticker is just under the sway bar about the 5 o'clock position on the diff cover.