About two months ago, I was about a mile from the house and stopped at a stop sign and the torque converter was just about locked up. I had to stand on the brakes to keep it from pulling itself across the intersection. I got to the house with it doing it the rest of the way there every time I had to stop or slow down and turning the truck off and back on it fixed itself.

Now the newest symptom is the engine really badly defueling on the shift from 3rd to 4th gear and sometimes being really slow to change even with the defueling. This morning, I started up and put it into reverse and it took it 5-7 seconds to actually shift into reverse and when it did it clunked in. I got to work and pulled into the parking place and put it in park. I sat for a second and put it into reverse again it shifted fine, but did it a second time and it was really hard to get it out of park the second time.

I have a Dr. Performance programmer and run it always in the tow safe program. I used it to clear the learning on the tranny hoping that would get it working right again, but nope.

What is happening and what should I do to fix it?

Well if on shifting I am guessing you mean you are on it and when it should shift it just stays there until you let off some and then it shifts. I that case look and changing fuel filter. Even if it is a few thousand change that and see if it helps there. If not one needs to know the fuel rail pressure at the time. If dropping off then maybe the fuel regulator is going bad. There is a shim kit that you search here and find at many different vendors here.

For the Transmission maybe a new NBSU will fix that. But no codes of any kind.

As to not shifting into reverse did it go forward if put in drive and still no reverse? If neother there has been know issue of converter drain back. Usually doesn;t happens over night but may depend if parked on an incline. Normal if it sets for a week. Not sure if there is actually a TSB from GM on this or if they fix it or tell you to live with it. But if this is the case then change torque converter to say one of the many vendors sell. No idea what would be one that can keep it close to stock. But Mike L can tell you.

Brian