no forward gears-solenoid? suggestions?

[sharlabarla]

I have a 99 Powerstroke, and it is an automatic. I'm getting really frustrated because I was told that I needed a shift solenoid, but can't seem to find the actual part that I need. I have found a few variations on it, but still I am not sure. The transmission will still go just fine when i put it all the way down in 1st and it works fine in reverse, just not drive...any suggestions before I take the truck to a mechanic?