un hook the linkage and try the selector shaft (molegrips or similar) if it moves ok then the linkage is at fault, if it dont you box is buggered

i had a xr2 box in my estate that needed bashing from left to right when goin up or down gears, it was interna (box had 3k on it and was like new)l but i put up with it for a fair while but it was annoying