Sounds like TCC shudder to me, but I'm not 100% convinced. You need to get your car scanned, and checked to see if the shift is being commanded or not. Any GM Tech2 device can do this, but a powertuner or HPtuner, or AutoTap works great in this instance for scanning? Where are you located??? Best bet if you don't have a friend with a scanner is to post on clubgp, and ask for someone in your area to do a quick scan for you. PM me, and I'll let you know what parameters need to be logged. Buy them a subway or a couple of beers as compensation. :-)

 

[align=left]Okay... I'm gonna be honest. I'm a total newb to this stuff, so I hardly understood half of that. It doesn't help that one will teach me 'cause I'm a chick lol.
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Find a shop that has a GM tech2 scan tool.