Trans seems to shift into neutral every time I stop in drive position. I have to select 1st or 2nd gear to get going again, then I can gently upshift to 3rd and then drive position. Next time I stop I have to start the procedure all over again. Also, when I hit passing gear, it goes to neutral and the RPMs go to redline.
It appears your transmission is on its way out and slipping . You need a transmission shop before you can’t make your vehicle move at all.
Avoid the chain type places like AMCCO but do use a local independent.
I would also check the engine and trans mounts. The engine may move far enough with bad mounts to yank your truck into neutral.