A833 Neutral Issue

Ok restated; all positions that cause a lock-up are ok. So if 4/3 and 2 cause a lock-up, those positions are OK.
Now;
if it won't go into first, most likely the brass is stuck on the brake, in a position that causes the brass teeth to butt into the slider teeth.
This is a simple fix; just put the slider into second to expose the first gear brass ring, and pry the first-gear brass off the first gear brake; problem solved.
Be careful to not overshift the slider all the way into Second gear because sometimes the struts pop up, and have to be coaxed back into their slots.
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This jam-up happens a lot when someone rams the trans thru the gears while both the input and the output are stationary. It will even happen after the trans is installed in the car. It will even happen when you come to a stop and try and put it into first gear, after the clutch disc has stopped spinning. In this case the usual solution is to put it into another gear, and put a lil clutch on it to break it loose, then go back and try first again.
When this happens repeatedly, it is because the brass and brake on first gear are working Too Good! I hate that, and so I take the trans apart and and swap that ring with Second. Then I take First gear over to the lathe and polish the brake smooth. So now, it takes a fraction longer to go into First at a stopsign, but at least I don't have to fiddle-fart with it. To make it even quicker, I grab First while the car is still moving ever so slowly, and I don't wait for the disc to stop spinning.
This problem usually only surfaces when you stop using EP oil and switch to ATF, or some mixture of ATF/EP. The EP gear oil is thick enough that it is a lil more difficult to squeeze out, which slows synchronization. But by the same token, the EP oil slows the cluster-gear down faster, increasing the likelihood of butting. so it's sortof a satch-22 situation.
I stopped using straight EP oil back in the 70s.