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P0750 is for the low/reverse solenoid in the solenoid pack inside the trans. This code sets when the voltage on the circuit is incorrect for a given state.
It could be an open circuit, short to power or ground on the low/reverse solenoid driver wire anywhere between the powertrain control module and the trans.
It could also be the powertrain control module itself. A wiring problem or the module are certainly possible but not common.
It could also be a connection issue at the large connector on the left side of the trans, but again that's not common.
By far the most common cause of a solenoid code is a failed solenoid, requiring solenoid pack replacement. The solenoid pack is mounted to the top of the valve body in the trans. Odds are replacing the solenoid pack will take care of your problem, but from what you wrote it sounds like the shop is wanting to replace it but not guarantee their work? That makes me a little uneasy. Before taking your $500 I would make sure they diagnose it correctly and rule out the wiring and module before replacing the solenoid pack. If they are unwilling to diagnose it correctly or guarantee their work then finding a more competent/confident shop is probably the way to go. It's not going to cost you any more money than that to have it replaced at a dealer and they have a transmission simulator that will rule out the wiring and module in minutes.
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I'm in the midwest, US.
The solenoid pack at your dealer will run $327. If you had one installed a trans shop you are either going to get an aftermarket one of questionable reliability, or you will get a Mopar part that the shop buys from a dealer and then charges markup.
The labor to replace the solenoid pack calls for 1.8 hours. You would multiply that by the hourly labor rate of the shop doing the work to get the dollar amount to expect for labor. The dealer I'm at is $75 an hour, so the labor total would be $135 at my shop.
Either place you get the job done you'll have about $30 in fluid and a bit extra if you replace the filters at the same time, which is a good idea. With the prices that I've just listed this job would be about $493 plus filters. That's right at the $500 mark that the trans shop quoted you.
Each dealer will have different policies regarding diagnostics. If you brought the truck to my dealer to have it diagnosed they would charge .5 in your case just to diagnose it. If you had it diagnosed and then we did the work then they normally woudln't charge the diagnostic fee, just the 1.8 for replacing the part. Not all dealers or shops will be this way though, they may charge you .5-1 hour to diagnose it plus the labor to replace the part.