adillac offered an optional "controlled differential". This is also known as a limited slip/ or posi-traction rear end. Unlike a regular rear end, that directs power to the wheel with least traction, a controlled/posi rear end directs power to the wheel with most traction...meaning better traction and stability.
As far as the gear ratios offered here is a list by body style and any options. The ID Number is stamped on the front face of the carrier assembly. See picture at bottom with blue arrow pointing to ID Stamp.
ID Number Body Style/Option Gear Ratio
2 reg. body style/non AC 2.94 - 1
1 reg. body style/ AC 3.21 - 1(optional on non-AC reg. body styles)
6 75 series body style 3.36 - 1
7 Commercial Chassis 3.77 - 1(optional on 75 Series body styles)
Posi/Limited Slip was optional and was denoted by a G in front of the ID Number on the front face of the carrier. You can also determine a limited slip/posi traction by blocking the front wheels, and lifting the entire rear end up in the middle at the differential. Spin one of the tires. If both spin in the same direction it is a controlled/posi rear end. If they do not both spin in same direction or spin in opposite direction it is a regular rear end.
Other tell-tell signs is the special controlled differential label on the under side of the trunk lid and there is usually a metal ring around the differential filler plug that indicates it is a controlled differential and uses special lubricant.
Different Speedometer Gears used:
-57 to 64 Hydramatic (75 series), part # 351 1353, axle ratio 3.36, 21 teeth
-57 to 64 Hydramatic (75 series & 62 to 64 CC), part # 351 1354, axle ratio 3.77, 23 teeth
-59 to 63 Hydramatic (except 75 series, CC) and 1964 62 series, part # 351 1629, 2.94, axle ratio 2.94, 18 teeth
-59 to 63 Hydramatic (except 75 series, CC) and 1964 62 series, part # 351 1630, axle ratio 3.21, 20 teeth
-64 Turbo Hydramatic , part # A148 1859 axle ratio 3.21, 40 teeth
-64 Turbo Hydramatic, part # B135 9271, axle ratio 2.94, 37 teeth
-64 Turbo Hydramatic, part # A136 2049, axle ratio 3.77, 42 teeth
Location of Rear End ID Tag. With the "1" it is a 3.21 rear end.
If it was preceded by a "G" it would indicate a controlled (limited slip/Posi) rear end. Here is a picture of one with the G1 Stamped: