Although they are not listed specifically on the SPID trunk label here are the most common rear axle codes for the Buick Turbo Regal.
1984 Buick Regal 8.5″ 3.42 Ratio axle
1985 Buick Regal 8.5″ 3.42 Ratio axle
1986 Buick Regal 8.5″ 3.42 Ratio axle
1987 Buick Regal 8.5″ 3.42 Ratio axle
The rear axle ratio, differential type, manufacturer, and build date information is stamped on the right axle tube (as viewed from the front of the car) on the forward side.
The first 3 digits indicate the axle code, the next one indicates manufacturer (B=Buick, O=Olds, C=Chevy, P=Pontiac), the next is Julian date it was built and the last digit the shift it was made on.
Limited slip rears had a tag on the differential cover that stated it need posi fluid. Most of the time that is usually missing, especially if the cover has ever been off. However, a “B” in the axle code (i.e., 2BX) usually indicates a posi.
The 7.5″ rears have a differential cover that is smooth and kind of a squared off oval shape.
The 8.5″ rears have a rounder shaped cover with a raised horizontal area in the middle.