Take a look here, as it has diagrams of the powerflow through the trans in each gear selection. http://www.allpar.com/mopar/transmissions/torqueflite-tom-hand.html Look at the difference between "Breakaway Drive" and "Manual Low" and also what it says about engine braking in manual low. Breakaway Drive means taking off in drive using automatic shifting. I have a feeling our answer is right here somewhere, but I just can't pin it down. The governor could keep it from shifting, but not from starting out in first. It would almost have to be a stuck 1-2 shift valve (Stuck in the second position)
I can't think of anything else that would cause it.
I'll PM Tracy and see if he has any further ideas.
(I just PM'd Tracy, and maybe he can tell us something I'm not thinking of)