Identifying the gear ratio of an AMC 20 is a bit tricky. Dana axles normally have a tag on the left of the diff cover with the gear ratio at the time of build. The AMC axles have a code that has to be looked up in a table. The code is stamped on the housing boss to the left of the diff cover right near the axle tube. Good part about the stamp is it tends to stay readable longer than the tag, but you have to know the code. Since there are conflicts between what a code means for a narrow track and what it means for a wide track, don't count solely on the code to identify an axle. Bring a tape measure or know the source of the part.

Specs

The R&P ratio ranges supported by the carriers are 2.73 and 3.31-5.13. The 5.13 gear ratio is not available for the Dana 30 though, so it is rare to see it. 4.56s are the lowest common ratio between the Dana 30 and AMC 20. 5.13s are available for the Dana 44.

CJ AMC 20

The CJs used two versions of the AMC 20, the narrow track from '76-'81 and the wide track from '82-'86. Both versions use keyed, two piece shaft/hub design and 5 on 5.5" wheel bolt pattern. Supplies of AMC 20s were exhausted in 1986 and some Dana 44s were used instead in the last of the CJ-7s.

From 1976-1978 a larger 11x2" drum brakes was used on the AMC 20. After 1978, a 10x1.75" drum was used.

SJ AMC 20

The Wagoneer and full size Cherokee also used a narrow and wide track version of the AMC 20. Both SJ versions used once piece shafts and 6 on 5.5" wheel bolt pattern. Since the length and bolt pattern is different from the CJ version, the SJ shafts are not compatible without resplining and redrilling. I've never heard of anyone going to the trouble.