I found this:

The 7.5 has more of a squared of shape to the cover and straight axles tubes that measure 2 3/4"
8.5 uses a round cover and the axle tubes step down from 3" to 2 3/4" right after they exit the pumpkin.

2wd width from WMS to WMS is 54.5"
4wd width from WMS to WMS is 59"
Zr2 width from WMS to WMS is 63"

S10 Rear end Ring Gear and Axle Sizes:
1983 to 1992 7.5" ring gear 26 spline axles
Available from 1988+ 7.625" ring gear 28 spline axles
Available from 1995 and up, 8.5" ring gear 30 spline axles, standard for 98+ ZR2's with rear discs

These are some of the axle RPO codes and should be the same for all GMs with either of these axles.

GU4=3.08
GU5=3.23
GU6=3.42
GT4=3.73
GT5=4.10
HC4=4.56
G80= locking differential

Rear axle codes can be found on a label attached to the left drivers side door lock post. The code appears as a number or letter/number combination in the 'Axle' column of the label. The axle can also be identified by the tag on the axle differential cover bolt.

Lots of info here: S10 site

I just found this in my 2000 Service Manual. Can someone with a spare rear S10 axle take a look and see if this works?


On top of the right axle tube, near the center is a series of letters and numbers (8): (the X's are the letters/numbers; the 1,2,3,4 under them are there to explain what they mean).

XXX X XXX X

1 2 3 4

(1) Rear Axle Ratio

ZTM = 3.08
ZTN, ZTU, ZTW, ZSY, ZA2, ZC2 = 3.42
ZTP, ZTR, ZTS, ZTZ, ZB2, ZD2 = 3.73
ZTT, ZF2 = 4.10
ZH2 = 4.56