If you're at Pismo, you want to be down around 10 pounds in that sand. What you're experiencing is rear end hop. You're axle is spinning and jumping because it's digging in and out of the sand.

Take your tires down to 10 pounds and it will stop. You want your tires to balloon out on the bottom when you're in sand, especially that sand. When I used to take my Jeep there I used to run 6 pounds in the tires.

Good Luck!