How to use a multimeter to test the resistance of my tongue?
I read online that this exact procedure should give an output of 70K Ohms


Wherever you read that, is simply wrong. There will be a wide range of possible readings, all valid, depending on how much saliva is present on that person's tongue at that moment.

And, of course, the readings will also be affected by the varying contact area and pressure of the probes on your tongue.


  What's up with my multimeter?


Likely nothing. The varying readings you mention on different ranges, will be due to the different current used by each measurement range. My results are different to yours, but that doesn't mean your multimeter is faulty.


  Or is my tongue just insanely resistant to electricity?


Try making your tongue wetter and repeat the test. You'll get a lower reading.