That is a great data source, but too bad it is only for Washington state. It would be nice to have the same data for all 57 states.
U-Haul publishes its own index of in- and out-migration. It would be interesting to see the three sources blended as they cover different cohorts. Moving vans are probably used most often by professional people whose move is paid for by their company. U-Haul trailers and trucks are probably used more by younger people without a house full of furniture who are moving without the benefit of a company paying for the move or maybe by lower-income people and families. Driver Licenses no doubt overlap with both.
BTW, I use the Tableau Software at work to perform analyses (same tool used on the web page you provided). It is an outstanding analytical tool.
A fuller database from the Census folks here -
http://www.census.gov/hhes/migration/data/acs/state-to-state.html
Harder to sort out the big picture though...