Letting it drain for several days if you can do without your truck is not a bad idea. Another thing you can do is to fill it up with cheap tranny fluid (but with the correct specs) and drive it a few miles or so, then drain and refill with a quality tranny fluid. Or if you do not mind spending the extra $ after your initial drain of the old fluid, refill with a quality tranny fluid of your choice from the get go, then repeat.