How to determine what ratio you have without removing the cover

[Irish_Alley]

[Gagecampbell82]

I found mine stamped on the passenger side axel tube facing the front. There mine said
Gm35 TUM G070     2
which in a link I found online had a pdf file of ratios it translated to a 2.73 open diff. The TUM was the only part I needed to identify it but the rest will tell you the plant it came from and such I do believe. Here is the link to the pdf I found. Im not 100% sure the link will upload correctly because of its length. But if not you can search gm axle code TUM and it should be the first link. It will say Mitchell 1 where the website usually is shown. Not trying to overstep just make it slightly easier on some people.  although I've never pulled the cover to check.
http://vintage.mitchell1.com/PClubData/chassis/ent82/V2D829004.pdf

[Irish_Alley]

From link, Nice find
http://vintage.mitchell1.com/PClubData/chassis/ent82/V2D829004.pdf

[Irish_Alley]

I made a spreadsheet years ago that calculates rpm tables based on tire size or actual diameter, axle ratios, and transmission gear ratios.  It also factors in transfer case, range box, or overdrive unit ratios into the RPM tables. 
I setup a table at the bottom that calculates speedometer gears and ratio adapters too if needed.
The archive contains two spreadsheets
*-an .XLS file for Microsoft Excel 1997/2000 or newer.
*-an .ODS file for the free Open Office/Libre Office suites calculated applet.

http://www.mediafire.com/download/zk4j8ymtctekx5r/RPM_Gear_Axle_Transmission_TransferCase.zip

[Austin4wd]

I use the factory RPO codes in the glove box

[Irish_Alley]

you cant just go by that due to the fact the vehicle is prob 30+ years old and if you dont know the original owner and all the history, it could of been changed at any point in its life

[hatzie]

I use the factory RPO codes in the glove box

That only works when the truck is equipped with optional ratio axles.
The axle ratios are not called out on the SPID when the truck is equipped with the base ratios.  The base ratio varies by year and model... 10, 20, 30, etc.  If you are lucky enough to find the build sheet in the seat springs the axle ratio is on that.