2000 gmc 5.7 auto in the drive gear goes to 30 mph then after revs and feels like in neutral and the tow/haul switch stopped coming on

Typical failure is fatigue of the two wire cable that runs from the switch to the steering column.

Search "silverado tow switch failure" and you'll find several how to repair videos.

could be the switch or modual.check wiring for worn through or earthing.

Check that the orange wire in the right side of your steering column is not cut or broken. It has 2 conductors in it. When they short together, it will turn the tow/haul function on or off each time. If they don't touch each other, the tow haul function won't activate.

When the transmission is in "tow" mode, the converter is unlocked. That causes the engine to run at a higher RPM but also since it's actually slipping internally (by design), it builds heat. If your radiator is the least bit internally obstructed or if you don't have a large enough transmission cooler (mounted externally), or, a combination of both items, everything will get hotter than it should be. Heat is especially bad for your transmission. The fluid chemically breaks down resulting in clutch slippage and burning.
If you do a lot of hauling, I'd get the largest trans cooler available making sure it's attached to the return side of the one inside the radiator and also change the fluid to a good grade of synthetic (resists heat damage far better than conventional fluid). Even if you think it's clean, I'd also flush out the radiator just to make sure.

This is going to be a internal issue with a faulty intermdeiate or direct clutch meaning a overhaul is needed.

You should be able to put in 4x4 while in drive less than 35mph. Mine had a switch to turn to engage or dis-engage.

it could be a filter in your transmission

near the throttle body there is an adjusting screw(flat head) for air fuel mixture, it's on a deep cavity sometimes with a cover, adjust counterclockwise will increase the idle.

Sounds to me like transmission issue might want to have this check.............

tow in tow/haul and use 4x4 when necessary..it is how the truck is designed ..... :) have a good day