I had the same problem with my truck when I changed out mt auto with a w56c, the trans had a little water in it so I drained and flushed the trans and refilled with Valvoline 75-90w with a bottle of lucas oil. At first I could only get 1st the my truck had other issues (coil, igniter, fuel lines, sending unit, gas tank and brake lines) so while I fixed everything else the truck sat around for about a week. Right after I changed the igniter and could start it again I had first gear......
my guess is that after sitting the lucas oil treatment freed up a stuck fork or something
. I would say to put in a bottle of it and see what happens.....
