Those guys have weak diagnostic skills
Very good mechanics know how to diagnose cars with no check engine light and no fault codes
I suspect there is a communication problem, you have a vibration at certain throttle positions and they want a fault code/check engine light? Did you offer to demonstrate the problem? With a failing torque converter clutch you will eventually get a fault detected by the computer but this is a $3,000 repair, I would expect the service department staff to be somewhat interested unless they didn’t understand what you were talking about.
didnt read all the responses to your original question, but if your having problems like that and symptoms happen when on a angle uphill\downhill, or rapid turns, check your fluid level before you spend money having shops diagnose it. i rebuild transmissions for a living the impala has a 4t65 transmission they do have there common problems but this isnt a common problem for it with a 3.4l engine, torque converter problems normally only occur in a 3.8 supercharged or 5.3 engines. it could be possible, best way to tell if its a converter problem after making sure fluid is full (check fluid while engine running and in park) when problem acts up hold brake petal jsut enough to set brake lights without actually slowing down but also maintain your speed , this will stop the computer from commanding lockup from the torque converter. (lockup from converter is really only engaged once at a cruising speed while in your final gear in this case 4th gear for this type of trans, this doesnt apply to all transmissions but in your case it does. But if you step on petal for passing gear it does disable lockup for a bit until it sees certain tps\speed positions) … but all in all for your engine\year the main problems you will probally see with the trans is a p1811 code that will not set a check engine light, sypmtoms are a long or unconsistent 1-2 shift, beside that main other problem would be basically losing 2nd gear, or car wont move forward until warm and in that case the forward piston\seal is flipping, both problems are due to the factory clearance and can be permanently fixed with a proper rebuild from a knowledgeable builder… hope this helps