It is jamming in 1st gear. I have already replaced all of the bushings on the linkage, and adjusted (and re-adjusted) the linkage at the shift box.
What is happening is, the dog on the shifter column is jumping out of the R-1 fork, inside the shift box, making it impossible to shift out of 1st. You can see what I mean in the picture below:
http://lymanhall.smugmug.com/Trucks/1953-Chevrolet-3100-Truck/i-CFJgbdM/0/L/0815140951-L.jpg This is what it looks like inside the shift box when it's stuck in 1st. You can see that 1st gear is engaged (top shift fork is moved to the left in the picture) and the dog has dropped down to engage in the 2-3 fork.
Looks to me like the box is worn out, but if anyone has any suggestions, I would be grateful for the help.
