Transmission seems out of commission

[cfan10]

Ok, I have a question about my 84 swb it has sat for a while without actually driving, but I would periodically start it up. Recently I had it pulled to my mom place, yesterday I put it in storage cannot afford two vehicles on the road right now.
So, here is the strange part of it, having the key turned to the on position. It will go in neutral and all gears without any issue but when trying to push the truck back to tow it, it seemed like it was still in park. What could be that cause of this? Has the transmission gone out from lack of use, there are no alarming oil spills underneath it. As always thanks in advance for any relevant advice.

84 c10 swb
305/th350 trans/4barrel carb