Silver Lake Sand Dunes is one of the reasons I bought a Jeep!
We've been up there a few times, worth the 5 hour drive every time...
Last July 4th, was really crowded, but was still a great time in the sand!!
Obtaining your ORV sticker and vouchers are straightforward processes.
Here is a link for the dnr info - it has all the official info:
If you're going with kids, I highly recommend http://www.silverlakerc.com/
They are very family friendly.