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
1 answer
go on the internet ask google for parts gmc guys they will tell you