My truck is blowing cold air out the back vents and after about a minute of driving, the front air conditioner starts slowly blowing heat until if feels like the heater is full blast.....people have said there is a door that may switch off and lets the heat in..How do i fix this??

There are a couple of different things that could cause the loss of heat. First make sure your radiator and overflow tank are full and at there specified levels. Run the engine till it reaches operating temperature( between 180 and 220 degrees). Feel the upper radiator hose and lover radiator hose , they should both be HOT , this lets you know your thermostat is working. If the top hose is hot and lower hose is cold. Replace the thermostat. If they are hot, check to see if both heater core hoses are hot ( 2 5/8 in black hoses heading into the firewall). If one side is hot and one side is cold, replace the heater core. If both are hot, you may have a broken cable or vacuum diaphragm that moves the direction control valve from hot to cold. Below are some example diagrams to help aid you in your diagnosis.