Jack up the rear, trans in neutral, spin the tires. Count driveshaft revolutions for one full turn of the tire. The LSD will make it easier... though, harder to turn the tire.

 

I tried that. My Surgrip may be worn as the tires rotate opposite ways until I do it quick when it grabs and they turn together. Right now since I know how far it's off, I don't have to worry about inadvertent speeding.

The old truck is serving me well even though it has a lot of things to fix. I suspect my a/c expansion coil is leaking in the dash as I charged the a/c and it has lost its charge but the dye I put in isn't showing up anywhere. The dash is severely cracked. Worse than most I've seen but Gorilla tape is doing a good job of holding it together. I don't know if it would survive pulling it to replace the coil. The ugly paint job is working, nobody is cutting me off in traffic. I had one darling cutting through traffic, but he stopped when he saw me rolling up.

I did find out why the spare is in the bed. The Flowmaster 44 exhaust doesn't leave enough roon between the pipes to store it on the under bed carrier.

 

You can drop the blower fan, and see one side of the evaporator core..... Might see you dye there. It would at least confirm where the leak is.