When engine gets warm, transmision gears disengage...why? Do i need new tranny?

Do not "pull tranny out, check all, clean etc and reinstall". That's a waste of time and money. When you tear it apart, you'll need to overhaul it. This will take a very clean working environment, a really good transmission technician, and high quality parts -- as well as a good deal of time to go through everything. This is why a replacement with a remanufactured unit (that has a warranty I'm sure) is a better idea, for time, money and guarantee. This, if you do determine overall that a problem is present due to an internal failure. A good diagnosis should come first.