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Hi guys! I've always been one to read forums, but have never actually posted... until now.

Recently I bought a 2013 GMC Sierra. It's 2wd, extended cab Texas Edition. Before this truck, I had a 2003 Silverado, 2wd extended cab. I bought it with 100k miles and sold it with 303k! That truck was nothing but good to me.

Anyway, here's the issue I'm having. My new truck is lowered 3/5 (was already on the truck when I bought it). It is the McGaughy's 3/5 flip kit. It is C-notched and the truck drives amazing, until you get between 50-55 while accelerating. Once you reach this speed, it will develop a slight vibration/shudder, but will quickly go away and become smooth again. It does not vibrate/shudder if you are doing say 70, and then let off the gas and coast back down to below 50 mph. It only does it under acceleration. The harder you accelerate, the harsher the viration. One other note that may be worth mentioning, around 80-100 mph, it has a very very slight driveline vibration. It's like a very high pitch fast vibration. It's almost unnoticable, however it is there. Then, as the speed reach around 105-110 (which is double the 50-55 mph vibration issue) the vibration comes back and is much worse than the 50-55 mph vibration. One time I just pushed on through the high-speed vibrration, and once I got over 110, the virbation went away and was perfectly smooth again. However when deaccelarating, once it got back down to between 110-105, it started vibrating again, even while not on the gas (unlike the 55mph vibration which does not vibrate under deacceleration). So far all I've done is installed the Belltech transmission spacer (about half an inch thick) between the tranny and the mount, and that did nothing to help.

Any ideas on what I should try next? This is my first lowered truck so I'm not too educated on driveline angles and all that fun stuff. I've done lots of research and it seems like I will need a pinion angle shim, but I'm not sure which shim to get. I did go ahead and order a 2.5 degree shim yesterday and it should be delivered within a week or so. The angle coming out of the tranny is damn near zero and going into the diff is also near zero (with the naked eye).

Of course the speed portion was tested while on an extremely open road with not a soul in site. Please don't come at me telling me what the laws are or what could happen at those speeds, etc. I'm well aware of what I'm capable of and the right/wrong place and time to do it.

Thanks, and I look forwrd to some responses!!
 

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I suspect your on the right track with a pinion angle shim. From factory the pinion has some angle to it, I don't remember what the factory angle is thoug.
 
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