I need help figuring out where to look for my numbers to decode my rearend. I have a 1978 Chevy 1/2 ton with a 12 bolt in it, but i was told they only came out with 10 bolt 8.5". Is this true? Any help would be appreciated.

The numbers (should) be found on one of the axle tubes, I say that because I have found the numbers on BOTH the driver, and passenger side tubes over the years all being the front side of the axle. Your axle is old enough that the numbers could be non-existing due to the elements rusting them non-readable, sometimes the stampings were not made deep enough to be completely readable on new units.

Your 12-bolt axle has a ring gear measuring 8.875 inch (8-7/8")
The pinion stem should have a measurement of 1.438 inches.
And your axleshafts have 30 splines.
This URL will show you what I already did AND give a pic of the cover for identification purposes also.
http://www.drivetrainsuperstores.co...nfo.asp?axle=87

With my experience with GM's truck division, they "started" switching over to the 10-bolt 8.5 inch "corporate" rear in 1977 and didn't use every one of them until after 1981, with some straggler units entering into production vehicles well into the mid 80's.
In 1977 GM also "started" switching the front axles in the 4 wheel drive trucks from the Dana 44 (1/2-ton), Dana 44HD (3/4-ton), and the Dana 60 (3/4 & 1-ton units), to the "corporate" 10-bolt 8.5 inch ring gear in the front of the 1/2 & 3/4-ton and the front of the 3/4-ton, and the introduction of the 9.5 inch 14-bolt for the 3/4-ton rear.
The Corporate 8.5 inch 10-bolt axles were used in the 1/2 & 3/4 ton 4WD front ends until the trucks switched to IFS in 1988 (not including the "V" series 4WD), and the Blazer/Suburban which switched in 1992 to the already existing IFS.
Dana remained the axle of choice in the 1-ton trucks until the introduction of the IFS front drive axles.

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Thanks M&M. I appreciate the info. I will look on that site to check it out.