I have had 2 shops look at this thing and niether were helpful or could figure it out.
The last shop cleaned my coil out inside the truck. It blows more air now but still has the same issue.
What it's doing is it runs cold air at idle and low speeds, but when I get up to hiway speeds, like 55-65 in 5th, it slowly looses cool air. The compressor never shuts off. When I slow down, it comes back.
Gauges show a perfect pressure at idle and at 2500 rpm.
Any lead would be helpful at this point. It's 101 with 90% humidity. I'm cooking everyday.

 

Is it overcharged? If there is too much refrigerant then the evaporator pressure may be too high at higher RPMs and isnt able to suck up heat.

I know you took it to two shops and I would guess they could look for something like this, but IDK if they would have a refrigerant weight for an 81 'yota and may have just eyeballed it.

Thats all I can come up with.

 

if its r12 get rid of it and put on a new h valve or oriface tube and a new dryer then recharge it with 134a freon ,,,