Did you put the side cover back together on the bench and test it? The 1-2 lever should be locked unless the 3-4 ball is exactly in the detent, and vice-versa.
Check that.
Your 1-2 and 3-4 sliders (the big round groove that the ends of the shift forks fit into) should be able to slide back and forth.

As far as lube, I would NOT use ATF, even thought some do. Get a 75w90 or 80w90 GL-4 (not GL-5) gear lube.
GL-4's are hard to find unless you look at Synthetics. Most all are GL-5 (formulated for rear-ends and not real brass-synchro compatable.

Redline makes a few, Pennsoil Synchromesh is good. Amsoil makes em too.