I do all my own maintenance, and own a diesel pickup. Transmission takes 20 quarts for a full flush/change. At $13/qt, plus my time, and everything else, it ends up less expensive to run it by the transmission shop and pay them to do it ($210 with tax). Plus they pull the pan, replace the filter and clean the screen.
I have done it several times. No drain plug on it, so I suck it out with a fluid evacuator before pulling the pan. A lot less mess that way.