1999 GMC Suburban revs high, but transmission won't slow the vehicle down

I ran into a strange problem has anyone had this. When going downhill at freeway speeds or coming to a stop, my engine revs high and does not engage the transmission to slow the vehicle down.

The problem might be caused by a faulty Idle Air Control valve.

The IAC valve controls the engines idle speed under all conditions whenever the accelerator is released.

If the IAC valve allows too much air past the closed throttle plate when accelerator is released, the O2 sensor detects this extra air and adds more fuel to the engine, which causes the engine to rev high.