my 87 quit once. It was the driven gear on the speedo adapter on the tcase. The pressed sleeve in the tcase output housing had walked forward and chewed up the driven gear. Don't worry about orientation on the driven gear. Count the driven gear teeth and align the mark with the right teeth count number on the housing. The driven gear shaft is offcenter in the housing, and you rotate the housing to get the right drive-driven gear clearances. Drain the whole tcase (you don't know what's in there, anyway) and pull the driven gear adapter, and the problem should be obvious if it's not the cable itself. Mine was "jumpy", too, but this could be snagging, unlubed cable or the gear train getting eaten up.