Taken from here: http://www.performancetrucks.net/forums ... ost5379058Thank you for your business.. We know the Y-Pipes are close to the drivelines on the 99-06 4wd trucks, but it should clear just fine with a good set of motor mounts. We actually just revised out 99-06 Y-Pipe last week to fit the 4L80E transmissions. At the same time we dropped it down a little for the 4wd guys, and added a O2 bung for wideband tuning as well. That prototype was just sent off. It will be at least 6-7 months before we have a batch of those on hand.
As for the headers, we have been selling those for the 1999-06 4wd trucks for several years successfully. The only thing that I can think of is that your Avalanche might have a little bit different chassis than the silverados, or maybe the front diff is a little different. I am honestly not sure. Either way if you are not able to make them work you can always send them back for a full refund.