Transmission stuck in Third Gear while driving

[highland62]

While driving (Forty miles from home)and attempting to down shift from third I could not move the shifter into second , it moved a fraction of the way and then refused to go further.
After removing the floor pan and shifter assembly and checking the shifter proper for operation I could see one of the three Plates (part #7350113) up on top of the sync ring and not in the notch where it belongs.It appeared to be worn away on the opposite end . After sliding the Sleeve(#8376245)back towards the Third gear position I was able to push the plate back and down into position and place the Tranny in second gear and drop the shifter back in .
We limped it back home using only First and second which provided more time(slower speeds) for looking in back yards for more Jeeps.
My question is what locks the Plates in place? or are they simply trapped between the Hub(#8331733) and the Sleeve(#8376245 )and can only be removed after the shaft is removed from the Tranny.
Thanks in advance