For the past five years, trades skills have been identified as the #1 shortage in surveys of manpower executives. I realize you asked for “one area,” the classification trade skills is rather broad.
Surveys of the NFIB (small business) indicate many job openings are going unfilled for lack of qualified people. (The area isn’t specified.)
The Conference Board index of internet job advertising is very high. This is broken down by category and location. Check it out:
https://www.conference-board.org/data/helpwantedonline.cfm
Since the economy crashed, people have been holding on to their old job for fear of not being able to find a new one. Their livelihood rests not in themselves and their willingness to work, but in the fortunes of their company. But, there are no guarantees. Companies are always changing. In a growing economy, people can adapt themselves to changes because while some jobs are disappearing others are opening. But not in this stagnant economy.
http://www.moneycrashers.com/what-is-national-us-unemployment-rate/
Your source, as with those who believe in other “zero sum” situations which don't exist (take the GOPe’s calculation that the GOP must appeal to illegal immigrants to win any election, as one example), leaves out a huge number of people that are available or swayable. Your source also leaves out anyone who is working in a temp or part-time job outside of their area of expertise to get by until something better shows up.
Do you have better numbers available?