It's the alpha-numeric code that designates the calibration for your vehicle's drivetrain. For example, I have one here in front of me for a '93 4.9 w/ 5-speed and the code is "T2D1". It's the largest digits on the ID tag.

Find out what is in your truck now and then search for a replacement either locally or on-line.

That said, I too wouldn't fully trust the diagnosis from your mechanic. Lots of perfectly fine PCM's have been replaced by "mechanics" who don't otherwise know how to do proper diagnostic work.