Shift Range Inhibited on my truck, What is it?

Hmmmmm, cryptic message last night.....

I was shuffling some trailers around, including my TT and I stuck it into drive to go, and found no drive, even though the indicator told me I was there.
Message in the odo screen read "Shift Range Inhibited"

I was in neutral even though I selected Drive. A little on the pedal, no go, truck isn't moving.

OK, what the flip???

Put it in Park, drop it back into drive and away I go. All is well.

Anyone else have this happen? I have no idea what it means or what the problem is.

I had this happen once a few years ago, same scenario. but a hundred thou miles less on the clock.

Any ideas?

Oh yeah, truck in sig