unplugged, remove the screws holding the shift lever plate ...and switch. old units have a short cast lever in a small panel. newer rod with ball end. you will see a rod protruding through a flat plate with u shaped slot. 1st is left and up. 2 is middle of the bottom and 3 is right and up. your shift lever moves that rod to the 3 positions. Yours will be in that upper right corner.
If the yoke on your lever is free to turn and engaging the rod, things are stuck inside. You may have hardened grease which means disassembly, cleaning and regrease. you may be able to move that rod with some tool. properly greased and adjusted, it's not that hard. You are probably not going to hurt anything applying some force.....no gorillas. everything pivots on hefty shafts and it's spring loaded. the little rod is on the end of what is called the tumbler which has the assorted gears. a spring holds the tumbler against the drive gear, and the rod pulls against the spring and allows the tumbler to move along a shaft into each gear position. tumbler slides forward and back on a keyed shaft. prying the rod down is working against the spring and pivoting slightly. Pry down the "U". It may release the goo, and running it in all gears may redistribute the grease. Since it runs OK in gear, chances are that nothing serious is broken inside. All of the parts are turning all of the time. It's just that different gears are meshed.