I perform QC reviews for our FHLB fixed rate loans. For reverification of income and employment, I've found it a lot more efficient to make a phone call rather then send a letter and wait for a response. Our institution usually relies on W2s and 2 recent pay stubs to verify income. Under the FHLB guidelines, its acceptable to reverify this information verbally as long as you document the time and date you called and the name and title of the person you talked to.
I usually ask for the HR or Payroll dept. I tell them my name and institution I'm with. I tell them that one of their employees recently obtained financing with our institution and as part of a Quality Control program, I just need to verify some of the information they provided during the application process. I give them a couple basic pieces of information, such as YTD income and ask them to verify it. As long as I'm giving them the info and they are just verifying it, I've never come across anyone who wasn't cooperative.
I like to make a phone call and be done with it rather than waiting weeks for a response in the mail that often never comes..