How do I find out which rear diff to get? No tag in glovebox to check

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Replacing the whole rear on my truck, and I need to know what rear differential to get. Unfortunately, my glove box was broken and taken out a few years ago, so it's just an open space to look at wires. Also the drive shaft is locked up, so I can't just spin the tires, and watch the shaft to figure it out. Any suggestions?