Polyurethane is a material with a variety of durometer readings (basicly how stiff it is). The "Wacky Wall Walker" toy is made of soft polyurethane, stiffer polyurethane is used in some portable telephones that are so stiff they break when dropped. The trick with polyurethane is quality control of the stiffness desired by the item designer. Spring spacers for example should be slightly soft to dampen vibrations (so they can replace the rubber spring isolator if desired). Polyurethane suspension bushings should just be slightly stiffer than the rubber bushings they replace, but are often very stiff - which is great if you are a racer, and bad if you drive on the road.
In fact there was a single batch of Jeep coil spring spacers made by Tera Mfg a few years ago that was too soft. The company stood behind their product and replaced them. If you run across any Jeep spring spacers that are defective (very unlikely) I'll bet you could still get them replaced.
As for the 1/2" rubber spacer/isolator used by Jeep, it has also been known to fail and deform, especially when used with stiff aftermarket springs and very heavy Jeeps with lots of mods.
Daystar also makes spring spacers for Jeeps, they make both 0.75" and 1.75" spacers for the front and rear of a Jeep. This allows you to tweek the ride hieght individually front and rear for cheap. Thier reccomendation (which I think is a good one) is that no more than two items (any combination of spacers and rubber isolator) be stacked on any single spring. Any more than that, and you risk having the stack "pop" out in the rocks when your Jeep is tilted to the side, collapsing the suspension on that corner of the Jeep, and possibly losing the spring (which I have seen happen on a ramp test).
Daystar makes polyurethane spring spacers in Black, Red, and Camouflage colors. I use a pair of 3/4" Black spacers on front of my 2003 Rubicon, to compensate for the exact amount of suspension droop caused by the weight of a front bumper, winch, and winch mounting plate on soft OME HD springs. Using just the two spring spacer sizes plus the 1/2" rubber isolator, and obeying the "no more than two" stacking rule, you can enjoy a variety of spring hieghts as follows:
1) Replace rubber with 0.75" spacer for 0.25" increase.
2) Add 0.75" spacer to rubber for 0.75" increase. (My combination.)
3) Replace rubber with two 0.75" spacers for 1" increase.
4) Replace rubber with 1.75" spacer for 1.25" increase.
5) Add 1.75" spacer to rubber for 1.75" increase.
6) Replace rubber with one 0.75" and one 1.75" spacers for 2.00" increase.
7) Replace rubber with two 1.75" spacers for 3.0" increase. *
* = Not reccomended because of the possibility of coil spring overcompression and damage.
In every case, for greater than 1" changes you need to swap out the shocks with longer ones UNLESS you are using the spacers purely to restore the original hieght that was lost due to extra wieght. Failing to do this will compromise spring droop - TJ suspensions are often limited in rebound solely by shock length.
I like the Daystar spacers because they are sold in pairs for $39.95 and are often available over-the-counter at the local 4WD Hardware retail store.
