I noticed a few other details today. When it is cold it drives normally and when the engine temp is normal it still behaves until the transmission gets warmer. Once I have been driving for a while and the transmission gets warmer it starts locking/unlocking whenever I stop for a light.
Just to try something, anything, until I can get it rebuilt, when it started unlocking/locking at a light I put it in 2nd, and it stopped doing it. I had to do that at every light to avoid it locking and unlocking and breaking something.

Does this still sound like the forward clutch, or could it possibly be something cheaper like a solenoid?
I am in the process of getting the money together for a rebuild but that is a lot to gather on short notice.

 

It sounds like the forward clutch.

 

It figures.
I'm calling the owner of a transmission shop tomorrow. He was out of town last week, and he is a friend of a friend of mine.

 

The transmission mechanic texted today. The forward drive drum/clutch was cracked. He sent a pic that I will try to post here.
He has it rebuilt and back in the truck, but on the road test he said it is still cycling between neutral and drive. He said that is what broke the drum and he is looking for the cause.

 

The mechanic texted me saying he found a valve that was sticking. He said that was what broke the drum. That is now fixed. He tested it today and will test it again in the morning when it is cold and as it warms up, but it is good to go now.
He also said I may have had a problem with a snap ring in 4th that I didn't even notice.

 

Glad you got it sorted out!