not sure what rear end i have? bought the truck used it is a 98 12v.

 

I believe the auto's have the dana 70 and manual's have the dana 80. Not sure on the duallys, but I assume all are D80s. Mine is also stamped on 2 sides of the housing.

Also if you find the BOM you can plug it into http://www2.dana.com/expertforms/demenu.aspx under Light Axle and then Build of Material. That will tell you what axle it is. BOM can typically be found here as well as model number. This is a front axle so it might be a bit different for the rear.

 

Frostytj pretty much hit the nail on the head. You can also check the tag under the hood if it is still there, mine was there to give me the bad news on my open rear end...