If it blows hot air all the time then it's probably a refrigerant leak.
I haven't checked the FSM but all auto A/Cs I've seen have a low-pressure cutout that turns off the compressor clutch when the pressure drops too low. That may be why your compressor is not kicking in.
If it only blows hot air when you are stopped, it could be it's just too damn hot and you need more air flow across the condenser - adjusting the fan setpoints or installing larger fans may help.
Pop the hood when the engine is hot and see if the radiator fans are running. One or more connectors may have gotten knocked loose.
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