Big Al1,
It is fairly simple to change tranny fluid. Locate and remove the return line from the transcooler.
Place return line into a large drain pain.
Start your engine and the tranny fluid will start to pump out into the recepticle. Have 11 quarts of tranny fluid ready and begin to pour into tranny as the old fluid is being pumped out.
If the tranny fluid is old you will see a color change when the new fluid begins to pump out( if your vehicle is new shut off the engine after the 11th quart). At this point shut off the engine and replace the return line. Drive your truck 5 or 6 miles and check the fluid level for your tranny.
This approach will remove all of the tranny fluid including fluid in the transcooler, torqueconverter, lines, ect.
If you were to just drain the tranny fluid from the drain plug on the pan, you would probably remove about three to four quarts of an 11 quart system.
Check out Amsoil's web site for a more detailed instruction.
I have done this procedure on three different vehicles and found it to be successful. Good luck