Agree it is likely a synchro, try the oils as suggested and replace the shifter bushings and adjust the shifter linkages, it makes a big difference.

About rebuilding the trans, an average mechanic with a manual can do it; but, a good used unit may be less expensive and faster.

IIRC, there is an aluminum and iron version of those transmissions. I don't know if they are interchangeable...anybody???

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