My truck shifts and runs great. However the 4L80E shudders/grabs when downshifting out of OD. It feels worse when towing. If you floor it to throttle shift it it's much better (maybe not perfect), but if you're light on throttle and it shifts itself, it really grabs. Scary sensation (noise and hesitation). I've had the valve body bored/rebuilt previous to this problem and all was well. Seems to have come into play after engine swap and isn't going away. To me it feels like the torque convertor doesn't unlock quick enough before downshift. Any ideas before tranny shop?

Local GMC dealer said there's a factory reflash/upgrade for my truck. My VIN never showed a reflash when I checked it. He said no guaratee, but he'd start there. They are the only dealer nearby I trust. Is this worth a try?

Local mechanic says he can plug in and tinker with 4L80E "values" with his computer tool to see if he can adjust away the problem.

Or should I bite the bullet and get a "perfomance reflash" from SSDiesel, Heath or Kennedy? Would these have the updated values the dealer's talking about? If it doesn't cure my ills, will another dealer flash overwite the custom programming?

I'm getting a headache!!:(