2006 Chevrolet Silverado

Transmission problem
2006 Chevy Silverado V8 Four Wheel Drive Automatic 68000 miles

I have a 2006 with 68k on it. The transmission quite working today. I was taking a turn to climb a more steeper hill. The truck went to shift down for more power and all of a sudden the truck hit the rev limiter. No load noses and its never done this before. I have drive third second and first. The only thing that works is first. And second will as well but acts funny and makes a horrible noise. Drive and third act as if its in neutral. Reverse works fine. I was wondering if this is a simple fix. Or could the transmission possibly be done for?
August 9, 2010.

Is the fluid full and clean? Also, have any warning light come on?

The transmission fluid is slightly low. Its almost in the cold zone while vehicle is running. Although no lights came on in the vehicle.

Aug 10, 2010.
When ATF gets hot, it expands. If it is in the cold area when hot, top it off and let me know if there is any change. If there isn't, either the front pump or torque converter may have gone bad. Before jumping to any conclusions, have the trans control module (TCM) checked for codes.

Let me know what you find.