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Go to GMPartsDirect.com, enter your VIN, and go to the suspension section. They have exploded parts view, and you can identify the bolt and nut. They may even have the specs on the bolt and nut. I would assume that the bolt is Grade 8 or higher.
If that does not work out, you could take the top bolt on the other shock and take it to Home Depot to get it sized it their thread sizing device.
There is a wire and connector on the top of the frame close to the top shock bolt on many of these trucks. A number of us have accidently disconnected that connector while fiddling around with the top bolt and nut of the shock. If you have the same wire and connector on your truck, be sure that that connector is connected before you lower the truck to the ground at the end of your shock job.I got extremely lucky, I took the bottom shock bolt out yesterday and when I reached up to put the top bolt I purchased locally through the mounting hole I felt something laying in the gap between the mounting tab and the frame, pulled it out and BAM my factory shock bolt was there. even had an original nut from as a spare is some of my spare nuts and bolts I keep that my GF probably wishes I would throw out. but I did tell her "see this is why I never throw good nuts and bolts away". she as none to thrilled to hear those words lmao.
There is a wire and connector on the top of the frame close to the top shock bolt on many of these trucks. A number of us have accidently disconnected that connector while fiddling around with the top bolt and nut of the shock. If you have the same wire and connector on your truck, be sure that that connector is connected before you lower the truck to the ground at the end of your shock job.
I believe that the wire goes to the speed sensor on the rear wheel.Any idea what the wire/connector goes to ?