It's a bit of work, but certainly the cheapest way to diagnose the axle - just switch them left-right. Then see if the problem changes sides. They are totally interchangeable.
Trailing arm bushings usually last a long time, but they are hard to diagnose with the spring pressure on them. best way to really check the suspension bushings is to compress the spring and take the load off everything, then it gets real easy to find looseness.
Of course, check the bearing and brake mountings also.

DG