78 Powerwagon Trans issue

I have a 1978 W150 powerwagon, 318, 4 speed, with 75,000 miles. The issue I'm having is when I went to go for a drive, the shifter is in neutral, won't go into gear, and I can't let the clutch out without the truck dieing. I can't find an answer on Google, was wondering if anyone here has run into the same issue.
Author: Pwrwgn