Sounds like the shifter gate is worn..
It should kind of snap from 1 gear to another gear.

Regarding linkage adjustment EZ to do..
Pull the rods out of the shifter gate, make sure the gate is in neutral. Next make sure each shifter lever (on the transmission) arm are in neutral. Then one-by-one attach each shifter rod while make sure all of the bottom tabs (shifter gate) are lined up. There should be an alignment hole in each tab, insert a drill or allen wrench to keep the tabs aligned through the holes. Carefully adjust each rod's length as required. Pull out the drill alignment/tool, EZ to tell if adjusted rite the lever will move smoothly left to rite and spring in/out of reverse.

Just my $0.015...