Just thinkin out loud here, I had a similar issue with my truck when it got stuck between high & low for a bit. The shifting mechanism meshes better when the drivetrain is in motion. Is the rear end jacked up with the wheels off the deck? Just thinkin you may haqve abetter shot at engaging the gear if you had one wheel on the ground, then spin the other wheel by hand and apply pressure to the shifter at the same time. Assuming nothing is missing pieces it may allow the gears to engage properly.

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P