Thomas,
You said that the sending units were replaced, did they replace the digital or analog sending units, because you have two different sending units for each reading.
I have always said that the dash gauges are for entertainment purposes only, but they should be more accurate than you described. I agree with Mike in that the dash ground circuit is the first place to check. It sounds like your shop is more interested in throwing parts at your problem than diagnosing it. Any "technician" can change sending units, but it requires a good mechanic to properly diagnose an electrical system to tell why it is not working properly.
Gerald