So it(the shifter) will not move all the way into the 3rd gear position, like something is binding, or keeping it from fully engaging?
That is definitely an internal issue. Possibly broken brass("blocking" ring/synchro ring), or the slider/coupler is damaged on the 4th gear end, which will keep it from sliding all the way rearward into third gear. This can happen
without effecting 4th, btw.
Another problem could be that the "dawgs" that tension the slider, and press against the synchro rings to stop the gear when shifting can be broken, or out of place, keeping the slider from engaging 3rd.
No matter what, looks like you're gonna be doing an overhaul on it...
You
can replace the 3-4 synchro hub, slider, and brass without
completely disassembling the trans, but it still needs to be removed, and the gear train separated from the case.
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