Gears & Axles - GM differential unknown

i have a GM differential and im not sure what it is. It has 3.42 gears and the part number is 25766294. Can anyone tell me any info on it?

picture,picture,picture !!!

http://baltimore.craigslist.org/pts/2994784744.html here is a link to my criagslist listing with both my rears on there one being the 3.73 and the 3.42