Sounds like a plugged evaporator drain. If you can get under the truck, you will see a tube from the firewall or HVAC box. Try to dislodge the obstruction by inserting the erase end of a pencil or similar blunt object. Some drains have a 90° rubber elbow on them so it may be necessary to get the truck on a lift.
The only other sloshing noise that I've run across was with an air pocket in the cooling system and you could hear the coolant through the heater core.