Problem solved! Thanks, guys.
Turns out the all-important right rear tire was about 5lbs low. Filled it, then had no place to go 4x4 until today when I decided to take it for a spin at the town recycling center. Put it in 4H and rode around the brush chipping area, shifted back into 2H on the fly and as soon as I eased up on the accelerator, bingo! the light went out.