Why do I get gurgling sounds when I turn off the engine

You might remove the part of the air box that is under the passenger side under the dash. This covers the A/C evaporator and the air box, for fresh air, can get clogged or the A/C drain can get clogged and water can collect in that area. I am not sure why it would make a gurgling noise.
The only thing I can think of that would involve a gurgling noise when you trun the truck off might be a throttle body control that uses engine coolant to work and is failing/clogging and making the gurgling noise. Sometimes a failing thermostat or radiator cap may make a gurgling noise.
The heater core may be failing. It still carries some fluid and pressure when you are not using the heat. When you remove the cover under the dash, you will be able to see if there is any fluid from heater core or water from backed up air box or clogged A/C drain.
That is the best guesses I can give you. Diagnosing gurgles and other noises is a tough one.
I hope this info helps.