So I changed my tranny fluid today. I couldn't get the fill plug off, so I just drained it (it looked like mud) and put roughly 3-3.2 quarts in through the shifter. As far as my gears poping out (1st is doing it now too smh), I am pretty sure it is the seat that's causing the issue. Mine is tearing up, and there were little pieces of rubber everywhere. The grease on the shift lever is also from 1985, so it was all chunky and hard, so that wasn't helping. I cleaned it up, put some new grease, and ordered a marlin crawler seat, so now I have to just wait for that!
On another note, I wonder if an exhaust leak can cause CEL code 12 to happen... cause I have one of those lol