20psi is still pretty high, try like 15. I've heard a lot of places require you to go down to a max of 15psi.
My personal experience: the first time i went on the beach i ran 18psi and it worked fine but i was getting in the pedal quite a bit and flinging sand to get going from a stop. All subsequent times i've run at 15psi and it is so much better, no tire spinning unless you want to, truck floats alot better rather than sinking. By the way i have 31x10.5's
I know this is a tire question not a 'tire pressure when in the sand question' but i wouldn't base anything off the performance of your truck in the sand at 20psi, go lower and you'll be happier.