I got it into four wheel drive but now if I try to take it off, my transmission doesn't engage and basically nothing happens, the engine just revs.

Our Sequoia has the push button 4wd control. That is how we engage and disengage the 4wd. The lever on the floor will just change it from 4hi, neutral and 4low. Try leaving the lever alone, and just push the button again. Hope that helps.