You might try doing a search under "transmission slipping" or something similar and see whether anyone else here had similar problems with their tranny that turned out not to be the mode selector swtich. I don't think this will throw a code so not sure a scanner would reveal anything, but I could be wrong.
The other apparent indicator of a mode selector switch fault is that when you get the clunk, the gear indicator light on the instrument panel does not indicate the correct gear: either it is out altogether or shows a gear other than the one you are in. If you notice this happening then the mode selector switch is probably the culprit
Good luck. Anyone else want to chime in here, feel free.