Hi Karebear,
Sunrise Safari can be booked 90 days in advance. It is offered on Tuesday and Saturday mornings and you must be a concierge guest the night before. You also will need admission to the AK park the day of the safari.
Please note they may impose a minimum night's stay for the sunrise safari. Haven't done it yet, but there is talk that it will be done that way.
I've heard they run several trucks sometimes so, I believe that will depend on attendance, so I don't think you'll have a problem. I would just book it as soon as you hit 90 days. That is what we did and we are a large party.
Oh, its $49.95 per adult and $24.95 for children 3-11.