Transmission slips when it gets hot

That is a common issue with that transmission as it wears. The seals and rings inside it are worn and as the fluid heats up those parts they start to slip as the fluid leaks past and the pressures drop. You could try a fluid change but the only true fix is to replace the worn parts. You might try something like the Lucas additive but like a fluid change it may do nothing.

The question then becomes is a rebuild of your current transmission or replacement with a used one or a re-manufactured unit justified. If the truck is in good shape and the engine is good they are all options. If it is getting rusted and starting to show other issues a salvage unit may be a better option just to keep it going a while longer.