It looks like I have the same transmission.
Apparently there is a spring and retainer that lift the shifter as you go from 1st gear to 2nd. The retainer isn't holding it in place, or something is missing? As you move the shifter from 1st to 2nd, the bottom of the shifter drops, and it won't slide to the other planetary gear.
Marking on the top of my case is T-75-148
Marking on right side of trans case is T-75-1 W-G-C (The "W-G-C" have dirt built up and is at an awkward spot, so it's difficult to tell whether they are the letter "C" or "G")
I'll send you a PM to work out any details.
Thanks for the help.