Jumping on this old thread... I'm currently trying to figure out what differential is in this 1974 GMC C6000 truck we're working on so I can get a new differential for it. The VIN is TCE624V556692 and the casting number is 987355. I've talked to just about everyone locally I can find and I'm losing sleep at night over this! If I can get ANY directives from someone who would have half an idea of what might be in this truck I would appreciate it! Thank you!