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**hatzie** · 10-16-2019, 12:10 PM

Mismatched front to rear axle ratios can and probably will do bad things to the transfer case and can foul up the transmission and/or one or both of the axles as well.

If you haven't put it in 4wd without a catastrophe you probably want to verify the front ratio.

3.73 is an optional 1/2ton axle ratio. Your SPID label in the glovebox should call out an RPO code for the axle ratio installed at the factory. GT4 is 3.73 axle gears and G80 is the Eaton Gov-Loc differential.
I'd be fairly confident the axles haven't been fooled with if the SPID matches the parts installed in your rear axle... but verification never hurts. You have to pull the axle cover to change the gear oil.

The G80 does bad things when used with limited slip additives so someone that knows will likely look at the SPID and assume the Eaton is still in place. If you put a limited slip diff in the axle it's a good idea to put some kind of tag on an axle cover bolt above the axle tube noting that Limited Slip Additive is now required.
This is one of those "posi" tags from GM.
https://www.ebay.com/i/330781133705?...d=330781133705

10-16-2019, 12:10 PM	#28
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,916	Re: Help with IDing rear diff Mismatched front to rear axle ratios can and probably will do bad things to the transfer case and can foul up the transmission and/or one or both of the axles as well. If you haven't put it in 4wd without a catastrophe you probably want to verify the front ratio. 3.73 is an optional 1/2ton axle ratio. Your SPID label in the glovebox should call out an RPO code for the axle ratio installed at the factory. GT4 is 3.73 axle gears and G80 is the Eaton Gov-Loc differential. I'd be fairly confident the axles haven't been fooled with if the SPID matches the parts installed in your rear axle... but verification never hurts. You have to pull the axle cover to change the gear oil. The G80 does bad things when used with limited slip additives so someone that knows will likely look at the SPID and assume the Eaton is still in place. If you put a limited slip diff in the axle it's a good idea to put some kind of tag on an axle cover bolt above the axle tube noting that Limited Slip Additive is now required. This is one of those "posi" tags from GM. https://www.ebay.com/i/330781133705?...d=330781133705 __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 10-16-2019 at 03:30 PM.