Hello Rap and thanks for your post.
Unfortunately, I'm going to have to be brief because, for some odd reason, Trulia's answer window and my computer are having problems. So here are you answers, very quickly.
Cracking in the concrete is not necessarily "sinking" but could be simply poor drainage around the 'on-grade' slab foundation that is being pulled apart by the expansive soil beneath the home. Texas, like California and many parts of the West, suffers from "Adobe" soil that is "expansive" (meaning, it holds water and "grows") and the result is visible and sometimes prominent cracking in the slab foundation. Unless Mesquite also has a high water table under the soil, the ground is probably rising (not sinking) in some areas and pulling laterally in others.
As the others have already noted, call either a foundation expert or structural engineer to help you. In most cases--at least here in California--unless the situation is gravely severe (in which case, we "jack up" the building), foundation or slab cracks can be repaired as simply as correcting drainage and epoxy filling the cracks.. However, only an expert can guide you in what is needed for the home.
Grace Morioka, SRES
Area Pro Realty
San Jose, CA