Not JUST the transmission. The maintenance schedule shows ALL drivetrain fluids every 15K... thats the rear axle, transfer case, transmission, and front axle.
There is no drain plug on the front axle! You have to suction it out with a pump.
Also, when you drain the transmission by just dropping the pan, 3/4 of the fluid capacity is NOT drained; most is left in the torque converter. A better choice would be a transmission 'flush'. Check your local shops for this service done with special flushing equipment.
When refilling the transmission with the engine off, add enough new ATF to bring the level up to the 'Full Hot' mark on the dipstick, then add 1 1/2 quarts. The final level needs to be checked HOT with the engine idling. To get the transmission up to temp, drive the truck at least 20 miles. Mild power braking at stops will also help the process. The correct temperature is reached when you can't hang on to the end of the dipstick with bare fingers. With the engine idling add ATF as necessary to reach the Full mark. Don't overfill. More than a 1/4" or so over on the stick will result in fluid foaming and poor trans performance.