I'm assuming you're talking about the bushing at the pitman arm.... that would be the draglink. If that's shot, start there and replace it. If the thud's gone... great! If not, start inspecting everything else that moves when you turn the steering wheel. Draglink at the tie rod, tie rod ends, track bar, etc. Have someone turn the steering wheel back and forth while the tires are on the ground and look for play in any of the tre's. Next thing to look at would be the axle shaft u joints.
Do you notice any vibrations, delayed steering, shakes, bumps, etc while driving?
There are a number of lube points on the stock suspension.... if I can find the graphic of them I'll post it up.
Edit:
Here ya go.... Marine proof'd so anyone can do it.
http://jeep.us41.org/lube/index.html
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