Ive never messed with a rear axle before besides the occasional fluid change but I'm nearing the back part of my truck and I'm about to redo the rear shoes, and rear bearings so while I have the axles out I said might as well stab a locker into there. The rear diff has been swapped out by the previous owner but he said it was a junkyard find with a better gear ratio. My truck is 1983 gmc with a 10 bolt rear diff. My question is I'm about to order the powertrax lock right from summit and I want to make sure I order the right one. I'm looking at 28 spline 8.5 inch powertrax model, Am I looking at wrong one or am I on right track? Is there something to look for in my diff to determine if it's a 7.5, 7.6, or 8.5. Inch? Do I have to pull the axle shafts to determine the spline? It's a 5 lug. Sorry for all the questions just want to make sure I order the right one the first time. Thanks in advance.