I never answer technical PM's as it subverts the whole idea of an open public forum
So this should be the schmatic of your box
View attachment 43567
The shift keys are the parts marked 8.
The shaft # 67 has 2 slots in it and the keys slide along it to select the gears.
Note the gears marked # 83, # 49 , # 50 etc have slots in them for the keys to fit in and lock that gear to the shaft.
Also note near the bottom, the brake rotor # 36 which is fixed to the layshaft # 4
If that rotor is not turning then the brake is locked which locks the diff as mentioned in the previous post.
Thanks Bert. I split the cases just now and didn't see anything broken in my very quick glance. I will take a much closer look at the #67 shaft and #8 keys. The brake rotor was spinning fine. So far, since I have gotten the case halves apart, I have run them through my parts washer and installed new output shaft seals. We'll get it going.
More to come!
Thanks again!!