Reply to "Doesn't want to stay in 3rd or 5th?"

Rocky; Just went through the same situation. I found that one of my U-joint yokes was not tight enough and permitted one of the spline shaft ends to partially pull out. Take the metal shift gate and rectangular cover off the shifter, which will act as a stopper in 3rd & 5th - test to see if you can then engage the gears. 2nd and 4th will also engage prior to where they once did in the shift gate. If so, then one of the yokes has probably let the spline end slip. Also check your shift linkage center support for tightness. As r mcall states, you've answered your own question, a linkage adjustment is probably required.
I found that my adjustment required each spline to almost be "bottomed-out" in each yoke, then a final adjustment at the adjustment section of the shift rod.
You will need a second person's help as they will SLOWLY move through the gear selection as you watch each yoke for linkage rod movement.