Taking it to a shop is the worst thing to do. The flush machines they use are ok IF the transmission has been flushed on a regular basis. The problem becomes apparent when you have a transmission that has not had the service done regularly and the process removes the particulate matter from the small nooks and crannies and redeposit's them in other areas due to the back flushing procedure and can cause blockage and burn up the transmission.
The best way and safest is to simply replace the fluid using the transmissions pump. Like mentioned, just simply dropping the pan and replacing the filter in not good enough. The pan hold about 5 quarts, but there is about 11 more quarts in the torque converter. So it is pointless to only change 1/3rd of the fluid in the system.
This method can be done by removing the cooler line at the top of the radiator on the passenger side. Then connect a brass barb fitting to the radiator that will accommodate a 3/8th's rubber hose about 3 feet long. Get a milk jug marked off in quart increments. Take the other end of the rubber hose and put it in the milk jug and have a buddy start the engine. This will begin to pump the old fluid out into the milk jug. Allow it to fill up to the 4 quart mark line. Turn off motor and pour in 4 quarts of fresh fluid into the dipstick tube. Simply repeat the process until you get fresh clean coming out into the milk jug.
If you really want to make this easy, add on an external filter. This way you will no longer need to crawl under the truck and change the internal filter and make a mess doing so. You can simply stand up and change the filter in about 20 seconds. This method will also aid in exchanging out all the fluid in the system by removing the input line at the filter adapter and sticking that hose in your milk jug. BTW I do this process every year.
Though I now use a hydraulic filter from NAPA p/n 1269 that is capable of filtering down to 10 microns that is not shown in this pic.
