Opinions wanted on shuddering TH-350 trans

Hi all,
Recently had the TH-350 in my 1975 Chevy truck rebuilt. (Was driving on the freeway, heard a "fweeeee!" and no power to wheels.) I'd have
done the job myself, but was quite a ways from home, so had it towed to a transmission shop local to the area.
Ever since the rebuild, there is a pretty heady shudder when you step on the throttle. It's worse at higher throttle angles and lower RPM. It shudders in all gears, but more noticeable in 1st and 2nd (really have to pay attention to feel 3rd). If I do a "stall test" there's no shudder. Only when the vehicle is accelerating. The tq converter is new and the fluid level is correct. The driveshaft is PROBABLY okay; at least it was when I brought the truck in in the first place!
Just want some pointers on what to look for before I pursue the matter with the credit card company. I'd like to not have to reverse the charges on a mom 'n pop shop.
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What the heck is Shuddering?

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