My wifes jimmy seems to be going in 4 lo even when she presses 4 hi. the light comes on the hi button but won't go over 20 without high revs. she said it goes into 4 lo and feels exactly the same as it does in hi. any ideas?

The Electronic Transfer case Case Encoder motor is defective.

Swap a working one in from a wreck in a pick and pull and you should be set.