A poly bushing is a bit of a toss up. It is stronger then rubber, by quite a bit. But it is much stiffer too - causing more bind and transmitting more vibrations. Also, it can squeek.
Certain horrible vehicles I have worked on (I once SAS'd a toy truck) ran poly bushings in the leaf springs. Greasable (ghetto) bolts were provided to help the poly flex, and to cut down the squeeking. Personally, I'm not into greasable (ghetto) bolts - even in sheer.
A friend of mine runs them in his rear links on the frame end. It still flexed well - but the supension design is what allowed that.
And I honestly have no idea how they managed to find that many bushings in a cherokee.
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