What should I do if the blower wheel assembly seems to be out of balance? 

Check the set screws on the blower assembly hub, If loose, follow this procedure. If the setscrew is loose on the opposite of the pulley side, untighten the set screw so that is not touching the shaft, then grab blower wheel assembly and pull the assembly against the leather washer and bearing, with palm of your hand shove the blower shaft inward ( leaving no gap ) then tighten set screw. For additional information see below:

CENTERING BLOWER WHEEL IN COOLER ON THE PULLEY SIDE

Locate end of shaft 1-7/8” from Bearing Assembly.
Pull Blower Assembly Hub against Bearing Assembly and tighten the Blower Set Screw.
Assemble Pulley onto Shaft and locate so it’s flush with edge of shaft.
Tighten Pulley Set Screw onto Shaft.