2001 chevy transmission problem

[Nightrider]

Hey guys hoping yall could help me with my truck.. I just bought a 2001 Silverado 5.3 with the 4l60e. It has 215k miles. They guy I bought it from sold it because it would not get out of 2nd gear. It was 2 quarts low of transmission fluid. So when you leave it in drive and hit 35mpg it will go like into nuetrol and Rev up I keep it on 2nd and it just keeps going.. I checked the wiring from the transmission to the main fuse box. Checked all the fuses. Been reading up on what people have tried youtube etc and still no luck.. Just wanting to try everything before I have to get a new transmission cause I would hate to have the same problem with a new one.. Any feedback would be greatly appreciated 😊