Sounds like an issue I used to have with my Vega (yep, Chevy Vega

) Waaaaayyyyy back in my teen years. The Synchros used to go bad and would not come out of third gear. I was able to dis-engage with clutch, but could not go into any other gear. Happened several times as it was an Opal trans. The very first time, I took the trans out, and put in on work bench to check it out. It rolled over and popped out of third. I came tothe conclusion it was "jammed" by something and re-installed it again. Ireplaced it2 days later [

]
I don't know if yours has the same issue, but is same description. Can you see if anything mechanical is in the way of linkage?
Jazzer