Well, that's a great observation, and worthy of contacting Eaton / Detroit directly. If you look at the online application catalog, you will see that the 913A589 is listed for the Dana 44 in the 77-79 Ford. But they specify the 913A591 for the 70-82 Ford. The interesting thing is that they show the Exact same Detroit Locker part number for both applications. So there is something special going on with the True Trac.
I was guessing that it has something to do with the stub axle in the Ford 80-96 Dana 44 IFS. I do know that they use the 913A589 in the jeep from 71-06, but the Chevy gets the 913A591 from 67-80. So it's worth a call.
So after a little bit of Web Sleuthing...it looks like the Truetrac is (sometimes) specified differently for front vs rear applications.
So if you have a Front Dana 44, you get the 913A591. If you run a D44 in the rear, you get the 913A589. General consensus is that the bias ratio for the front is reduced so that it is less noticeable. Evidently there are many people that run the "tighter" rear truetrac in the front. The Eaton application guide is written for children...and assumes that you can't differentiate the difference between a front and a rear axle.
There is also a group of web detectives that believe the helix on the worm gears inside the truetrac have a different profile from front to rear. Much like a ring and pinion, it is entirely possible that there is a "drive" side, and a "coast" side of the worm gear. If this is true, then the truetrac will behave "differently" in forward vs reverse. Also, much like your ring and pinion...you can't tell there is a difference between forward and reverse. You think that your axle gears work the same forward and backwards. But it doesn't. In reverse, you run on the coast side of the tooth. If you put a low pinion axle in the front...like the Early Bronco 44...you run on the coast side of the tooth when driving forward. It's weaker, and it puts more load on the smaller pinion bearing, but few people notice.
The only way for me to KNOW whether the TrueTrac runs a reverse helix angle, or just a different worm ratio is for me to have both of them open on a bench side by side. (and that won't happen any time soon.) The marketing literature from TracTech / Eaton / Detroit is sufficiently obscure, such that no reasonable person can divine this information.
This is usually the time when I stop caring, and just let it go. But hopefully someone that knows more about the truetrac can chime in.