Another method is to look at the B-Pillar label. I believe the only way for a 3.73 and/or eLSD is through the HD Tow. Everything else non-HD has 3.31.
Taken from another discussion thread:
Just below the bar code, you should see the word AXLE and a two-digit code (code 10=Non-Limited slip diff with a 3.15 gear ratio, code 15= Non-Limited slip with a 3.31 gear ratio, code=3L indicates a limited slip diff with a 3.73 gear ratio).