How do I find out which rear diff to get? No tag in glovebox to check on 1990 GMC 1500 Pickup

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?

Asked by for the 1990 GMC 1500 Pickup
go on the internet ask google for parts gmc guys they will tell you