First make sure everything is in neutral, because the 3-4 being in a gear will lock out 1-2.
You can take the cover off while it's in the car and should be able to slide the rings the forks ride in and engage and disengage each gear if you need to chk the trans.
The problem is usually in the cover, and to do with the lockouts between the rods in the cover.
The two parts I circled called "Interlock" that does this.
This is a different trans but the parts work the same, except they are inside the side cover between the shift rods that the linkgage plates bolt onto on yours.
