We use a mail forwarding service in SD - My Home Address out of Emery, SD.
We timed our paperwork so we didn't have to buy two sets of license plates for everything. I don't know if it's 60 days or 90 days... but you buy the vehicle plates and then have so many days to physically get there to take your D/L test and turn in your old license. (Be very sure that you have nothing on your record - fines, EPA testing... that would make the SD D/L facility take your license away without issuing a new SD license to you. )
We had lived in Illinois 10 years and moved to a new home in Michigan in Dec 2001 so we did not get our vehicles tested in IL in Nov ... and then when we arrived at the SD D/L in 2005 realized that it's important to return that little postcard with the box checked that says "Moved out of State". SD wouldn't issue a SD D/L until it was cleared up. We actually had sold the vehicle that had the hold on it... took 3 days to prove that to IL so that SD would issue a new D/L.
The mail forwarding service you pick needs to be able to handle the renewal of your license plates. We've been on the road for over 10 years now... with the same mail forwarding service. They do more than forward mail.