x2 on what James and others say, top hat style bush with greasable bolts. Never had an issue in 5 years on and off road, not noticed them squeaking much, or maybe their drowned out by all the other noises. Mine are poly.
Most important thing on shackle bushings is not to tighten them too tight, 30ftlbs is plenty, allowing the shackle to move freely, use nyloc nuts, none of mine have ever come loose even though they are not that 'tight'. Tight shackles will give rock hard ride and no articulation.
Hope this helps