[MJ4BTA]

It may only be a bandaid over other heat related problems, but the JKS upper shock conversions will get the bushings out of the engine compartment. There are better weld in versions, but quick bolt in would give you quick answers. I had to do them with my Cummins 4BT Comanche when I had the single turbo on it because the exhaust housing was nearly touching the top pin on the shock.
If there is enough heat in your engine compartment to degrade rubber or poly shock bushings daily, you are going to have other things melting soon. Do you have any vents in the hood? What type of exhaust? As in converter, restrictive muffer?
With no intentions to be rude, are you very sure its not the coils just pulling them to death? Bushings will soften with heat, but they shouldn't fail that quick.
The reason I ask is my L33 powered XJ never has horrible heat in the engine compartment. Now granted, I have a cowl hood, but i have headers, converter, huge flowmaster muffler and full exhaust to the rear bumper. My PCM is just as close to the passenger header as the shock mount hole is, with my coolant overflow rubber hose thru the hole and my air locker line, which is plastic, is thru the drivers shock upper mount hole. In fact, I have A/C hoses running 2 inches closer to the header on the right side than the shock mount hole, with no heat shielding, for over a year. And, thats with a lean running factory tune.
If you have enuff radiant exhaust heat to kill upper shock bushings, that are 4-5 inches away, you should nearly be able to see heat degradation on the spark plug boots by now.
I hope you get this figured out. This is a strange failure, Please keep us posted on what you find.