The work of God is much more extensive than merely preaching the gospel to the public. We have gotten into the habit of using that term "work of God" far too narrowly.
This particular verse indicates
1) God is working. He is actively, continuously, and personally involved in our lives.
2) His work is more widespread than first appears to the casual observer.
He will do whatever it takes. He is not an assembly-line worker doing the same things over and over again. He accommodates for the way things are going within the purpose that He is accomplishing.
Salvation is a term that the Bible uses quite broadly. It literally means "deliverance," but it can be used to include anything that God does in His efforts to bring mankind into His Kingdom. The "feeding of the flock" is His work too. As Jesus stated in John 5:17, "My Father has been working until now, and I work."
"Feeding the flock" is a part of His work, just as getting Israel out of Egypt under Moses was a work. The major emphasis, though, was different. Getting Israel into the Promised Land under Joshua was also the "work of God." But, again, the emphasis of the work of God changed. Organizing Israel into a nation under David was part of His work, but again there was a shifting of gears in "the work of God."
Rebuilding the Temple under Ezra was a "work of God" done through men, but the emphasis changed again. The rebuilding of the wall under Nehemiah a little later was also the "work of God." The building of the ark through Noah was the "work of God" at that time. The examples are almost endless, and so the conclusion is that the specific application of the "work of God" can vary from era to era.
John W. Ritenbaugh
What Is the Work of God Now? (Part 4)
Other Forerunner Commentary entries containing Psalms 74:12:
1 Kings 19:10-18