Re: North Padre Island National Seashore
I've been down to 25psi in soft light sand, that was with Ds in my SMB. If I reacall the sand on North Padre was thicker grittier stuff, whereas on South Padre (where all the development is) fine and whiter.
I've been to North Padre Island twice, first in my 2wd 79 G20 and later in a Jeep Cherokee... before I knew anything about sand, airing down or 4x4. Down by the surf the sand was really packed and any light vehicle could drive on it, but to get up on the dune out of the water past the first few miles you need 4x4 (it's not legal to go past the first few miles without 4x4 which is why I bought the Jeep Cherokee in the first place... )
I would make sure you have a winch and some kind of anchor that will work in the sand. Otherwise I'm a big fan of airing waaay down. I've been known to run in the single digits in my Cherokee (now that I know what I'm doing) which would translate to something like 15-20 in a vehicle the side of a van... probably not advisable without beadlocks.
All that said, it depends also on your tires... what brand size and rating are you running? How wide are your rims?