Do an earth test first. I used a jump lead from the battery neg to the starter mounting bolt. That showed a poor earth. I replaced both front and rear earth straps (1993 4/4 with cvh). Clean all contacts at the motor.
Next try cleaning the starter especially if it has the piggyback solenoid on the starter body. It works by energising a lever that moves the motor gear to engage the flywheel ring teeth. With age it can get sticky. You and bench test the motor with battery connections if needed. It may also be a case of worn brushes in the motor. A DIY job or take it to an auto electric repairer.
