If I understand you right
this gearset is a regular A833 gearset.
Is it used or new?
But before you go any further;
remove the driveshaft from the rear end.
If you have a spare rear yoke, drop the driveshaft and stick the yoke in there. If you don't have a spare, block the driveshaft up in a cradle so it has to stay there.
Now repeat your test.
Here's the why of it;
What you are describing is in all likelyhood originating in the rear brakes, especially if they are drums. Drum brakes are self-energizing. And in reverse, the shoes like to drag on the drums. If they start dragging, they will rotate slightly and then jam up.
Therefore;
A second option instead of removing the driveshaft, is to remove the wheels and drums.
If it should happen that this does not cure your problem, then I can tell you what I think could be wrong with that synchro; I have built hundreds of manual transmissions possibly thousands of them. But first, you need to prove that it ain't the brakes.