I didn't catch where you need the shock absorber—whether the wheel or the hitch.
That said, if you haven't rebuilt the hitch yet, consider this:
from Figue Trailer Axle Sizes R and P Carriages Seneca, Il
Specifically the cut-away center in the middle drawing. Nothing more than a square tube, a square bar, and four rubber bungee cords. Rubber does both springing and shock absorbing in a single material.