@Brad S. ,
I was curious what the solution to the foundation distress turned out to be. From my experience, if an engineer is going to put his seal (his career and liability) on the line, the engineer will want to see how the foundation behaves over time given different conditions (i.e. after wet and dry spells). Also, you may or may not need to measure the differential movement. Certainly, if you were to monitor over time, you would need to establish a benchmark. But if want someone to look at it once, snap fingers and give an answer, what do you think the measurements would tell you at that point? What if the differential movement was 3 inches instead of 2.5 inches, for example? Would that somehow change the diagnosis? When you pay and engineer a couple hundred bucks to come out and look at your structure, how much time do you expect that engineer to dedicate to the analysis?
I raise these questions because so many times people just say "get an engineer", but often times, getting an engineer is not the magic pill that is expected. If you want a full survey of the distress that is related to expansion and contraction of clay soils, you need to mobilize a drill rig and take samples. Then you need to do lab tests on those samples. You also need to do a floor slab survey AND monitor over several wet and dry periods. This procedure is done on commercial structures but I have never seen a home owner or residential investor have the kind of money needed to do it right.
But the solution for expansive clays will usually be the same. You can do some warranty foundation repair that will generally require multiple warranty calls over the next several years to keep shimming the remedial piers, or you will need to do chemical injection which is also not guaranteed to work the first time no matter what the contractor tells you. If you rely on the warranty, make sure look for the foundation company that has been around for 30 years and will be around for another 30 years rather than some nobody with a jack on Craigslist.
If you monitor with time, you might be able to identify surface drainage issues that weren't readily apparent on your first peak. Here you might find a solution that doesn't require as extensive of a repair. Or, at a minimum, you will find something that needs to be addressed along with the repair so that the repair remains effective over time.
Bottom line is; you should know what you are doing with foundations or have a trusted and experienced foundation guy (either engineer or foundation repair contractor) that can help you decide what the risks of a given situation are. But don't think its like calling a plumber who has total control over his working media.