1993 Mitsubishi Triton mj 4d56

I have a gearbox problem at the moment with my 1993 Mitsubishi Triton 2.5lt diesel 2wd. I bought it a couple of weeks ago, i knew when i bought it that 3rd and 4th gear were rarely able to be selected. Is this a common problem and can this be fixed or have to be replaced. If so would a gearbox from a petrol series same model be compatable. Or is their another match eg.. sigma, mazda ect.
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