here is a little test for you...

disconnect the vacuum hose going to the trans... its at the top
of the fuel pump.... leave the one dangling from the tranny and put
a plug in the other one... (no vac to the tranny is the goal.) start it up
and drive it around the block and see what happens....

if the thing drives and shifts right away then the problem is in the vac system...

usually when you have a reverse problem it means you need to rebuild the trans...