I have a 1997 k1500 with 4l60 I was coming home the other day going down the highway and my truck just acted like it was in neutral. I pulled over let my truck cool off and it has reverse park 1st and second no weird noises or any slippage in those gears it will shift to 3rd sometimes but when it does it acts like it’s in neutral if I stop it’s starts off in second gear if I have it in drive. It will drive fine if I leave it in Manuel 2nd gear as long as it doesn’t shift up to 3rd

 

Have you maintained your transmission with new fluid and filter?

 

You may be having issues with the 3rd clutch. How does the trans fluid look and smell? If it smells like hot brakes or burned clutch material you may have an internal issue. With defaulting to 2nd gear start you can check for transmission codes.