Dig under retaining wall and jack it up then pour new concrete
underneath. May not help much as the crack will still be there but
might only be a hairline crack at that point. This is the cheap and
easy way.

Other option is to demo the wall, set up some forms, rebar and pour a
new retaining wall. This usually isn't feasible since most walls are
higher but a 3-4 foot wall is a piece of cake. Once the wall is taken
out, you need to drive some rebar/anchors into the remaining walls that
stick out about 4+ inches so that the new concrete will attach to the
old wall.