I'm at a loss, so are the 4 local mechanics I talked to, and so is the chevy dealership.

It's a 2004 Chevy Silverado 1500 5.3 engine, 4x4, 4l60E, Z71 with g80 Diff, 170k Miles

All fluids and grease ports have been checked today and changed in the last 30k miles (oil every 5k), we can start the truck right up and drive but as soon as we get out of 1st gear we have to keep our foot in the gas. If we take our foot off the gas there is a very quiet grinding noise as the truck starts to slow down from somewhere behind the driver's seat and we can't accelerate. It feels like the truck is in neutral because you can press the accelerator and the engine revs but that's it until you get down to about 10-15 mph then it responds to the accelerator like nothing happened and the grinding goes away. No codes that the snap-on scanner can find, it first happened on the freeway when they were driving at 80 mph, they limped it home at 10mph scared to go any faster. The mechanics test drove it and experienced the exact same thing but they can't find anything wrong.

We know that the shift from 1st to 2nd is where the issue starts because of one mechanic's test drive. It up-shifts perfectly though all the gears but if you take your foot off the gas it feels like it goes into neutral. What do you guy think?

note: I also asked this on Reddit bc at this point the other driver is ready to just take the truck to the crusher but im not quite ready to give up on it.