P0742 on my 2000 Impala

My wife's 2000 Impala makes a whirring noise when it's driven on the freeway over 50 mph. Never when driving on the residential roadways. She drove it
to D.C. recently and, in the middle of Pennsylvania, the check engine light came on. (OH Great!). I told her to take it to an auto parts store that would check the code and it came back as P0742 (Something about the Torque converter switch stuck on). We took it to a local dealership upon her return to diagnose and they told me that there was no 4th gear any longer. Possibly could be fixed for a tidy sum of $450 but they would not know until they get it apart. If the entire transmission needed rebuilt, it would cost $2100, or $4500 if it needed replacement.
I talked to a transmission shop and they told me, obviously without seeing it, that (if the dealership was correct) and it needed a clutch replacement to return 4th gear, they would rebuild the whole thing (including updates - whatever they are) for $1700. They agreed that if it was only 4th gear, I could drive it in 3rd as long as I didn't go too fast. I'd keep it under 45 mph myself, but I can seem to pry my wife's foot out of the carburetor (oops, I mean fuel injector pump). She drove over 60mph in a 45 zone while going to drop it off at the dealer.
I asked her, "Why do you drive so *&^%$*& fast?" "I was only going 45 or 50."
That normally results in the "whatever" response, at least that's how it works with me.
I guess she keeps forgetting that the device on the dash tells what speed she is going. I dont think she looks at it very often.
ANYWAY.....Back to the subject.
I'm not one to pay thousands of bucks to have repairs made uneducated. I'm no stranger to taking engines and transmissions apart and putting them back together again so I imagine that I take this one too. I just wanted to check the board here to see if someone else had a similar problem and what they finally ended up doing. If it takes me 4 weeks this fall or winter, it would be like getting $500 a week to do it myself
Thanks,
CJ
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If you google that code there is plenty of info out there.

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IF, the torque conv. lockup clutch is stuck on the vehicle will die every time you stop..*stuck on!* meaning torque conv. now = direct drive......check your tach in the inst. cluster and watch for gear changes, count gears. their are other codes for no shifts from gear to gear ya know.........many things will cause this code, P0742, (poor grounds, poor wiring, poor connections/all GMs forte)............if it were me i would let anyone EXCEPT a GM dealer tackle this problem!...and, torque conv. lockup is NOT a gear. just a very small fuel saving at highway speed and a great inconvenience in city driving........as far as disassembly in the driveway. no way! these things are now a maze of spacers, clutchs, solenoids and critical tolerances that i wouldnt even attempt anymore outside a cleanroom. if you do, and are successful, my hats off sir!.........kjun
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