I had a quick chat with the guy who is doing the rebuilding of my 88 Getrag and he told me that this transmission had already been opened and the output shaft disassembled, because the first rebuilder had inverted the 1st and 5th pin bearings, probably causing the breakage of one of the bearing.
So I am looking for maybe both bearings, GM parts are 10051601 (1st) and 9441543 (5th) or a complete output shaft.
They are parts 39 and 50 in the Muncie Product Service Training Guide.