Don't know that you can assume they would be shot just because of age. Back in 2004 when I replaced the bushings on my '90 I was surprised what good shape they were in for a daily driven central Missouri car. That being said, I did mine with the Mazdacomp bushings since there was not an affordable Miata poly option such as Powerflex at the time. Do have their diff bushings in the diff that is currently on the car. With poly, and here will come the rimshots from the gallery, use quality lube and lots of it. And if it is Powerflex lube keep the nitrile gloves on and the shop rags handy. That is some tenancious stuff.