You will be tall enough. My stock hieght 2002 F-250 with a flag mounted on the tow hooks was tall enough. Just remember to air down your tires prior to entering the dunes. I would say some where around 15-20psi should be good.
Has to be orange flag, ractangular shape, the flag can have writing on it but it must be orange. You can get real nice ones for around 20-35 dollars at the store just outside the gate. They have a heavy duty fiberglass mast with the correct flag on it. I have several in my barn from previous trips.
Also if you are going on the weekend you must get a voucher from the voucher station by the dunes. The Station is open till midnight on the weekends. With out the voucher you will not be let in on the dunes. Also get there early so you are gaurenteed to get it in. If you go during the week no voucher is necessary.
The link above is all the information for silver lake sand dunes.