I'm on the prowl for a GM posi-Traction differential to put in my restored 1990 Silverado 1/2 ton. It has a 305 ci engine. A rebuilt 700R4 trans and the standard differential with 3:42 gears. When ever i go looking for a rear end there are 28 spline or 31 spline and some info about the rear end, but I don't understand what I'm looking at. Me I don't know one if it bit me. In the glove box is the RPO codes, I looked them all down and some make sense and my truck has one C5G=GVW rating of 5600 Lbs. and codeGU6 Rear axle, 3:42 gears Which i want to keep. Am I correct that with the right posi differential I'd only have to change the ring gear, because the pinion gear stays right where it is, or do I need to get the entire differential built by a shop ? That's the only information I have to go hunting for a Posi-Traction differential. Any advice is most appreciated. Thank you !