(Mods, I'm posting this here instead of General because this is where the OP was)
For awhile now, one of my 4-speed Mopars has had an intermittent problem where I couldn't get 1st or 2nd gear after pulling the shifter out of Reverse. Tried the adjusting pin and moving the Rev lever forward/backward, no luck. Someone, and I'm sorry I can't recall who, posted that loosening the side cover and tapping it up had worked for him.
Got under the car tonight after experiencing the problem, followed the recommendation, and the it worked perfectly! Even made the shifter work more smoothly, and it wasn't a big adjustment, almost imperceptible.
Whoever you are, thank you, thank you, if you were nearby, there'd be some beer (beer's like money in northern Idaho) for ya!