I've had a off/on problem with my ABS light, last time it was the front drivers side sensor, the gap was off, pulled it out, cleaned and put back in, no light since.
Here is a link to a troubleshooting guide
Page 77 has the 798 code.
Ask the service manager if they have check the resistance on the circuits as described in the manual.
Bendix has a pretty good diagnostic system. Even on my 2002 I can retrieve the blink codes. The new systems have an interface to laptop that have more capability to diagnose so my guess is that your shop is providing you accurate diagnostics. Have them show you this, have them run the diagnostics while your standing there. It would be hard to fool you then.