How can this be fixed? Can they be shimmed or do they have to be replaced?

First of all check the 36mm nut to see its torqued enough (250ftlb)
Is it IRS?? Cant remember if there were 1500 models still in 71. Either way you can replace the bearing/s.
If its IRS there is two, an inner and outer with a spacer in between. Spacer needs checking for wear too.
Swing axle has only one bearing.