You might think the stock sized stuff is enough but unless you can lock the 40s up at 50 when the brakes are soaking wet then you really don't have "enough" braking.
I haven't driven my dads vehicle in the rain but based on the dry performance that is better than most factory vehicles, he has the best bang for the buck out there. He has calipers that most wouldn't buy and aren't needed but between the pads and rotors, you might have an extra $150 in it than your standard 1/2 ton setup on all 4 corners.
And Erin, brakes on the rear axle DO matter.... When you're in 4 wheel drive everything works together. When loading up the torque converter in low range, every bit helps and matters.