unless the PO swapped in a 80.
the 70's cover is almost as wide as it is tall and the 80 is alot taller than it is wide.

 

Check in the glovebox. There is a piece of paper there to tell you. Or go to a dealer and they can do a vin# look up.

 

If I'm not mistake the axle should also have what it is stamped on the third member forward of the axle tube. Let me do some checking and get back to you on that.

There should be a metal tag between two bolts on the cover that will have your Bill of Materials #. That is unless the PO got rid of it.

 

mine has it under the hood,check there

 

If you are still having trouble crawl under the truck and check the front side of the differential housing. Mine is cast into the housing on the upper right side.

 

take a picture of the differental and post it up here, we can tell ya what it is just by looking at it

 

The " with 9 1/4 inch ring gear" is the 1/2-ton rear end.