My experience with "Shift Kits"
I currently own a stock 95 Explorer, but in years gone by I owned a Chevy Malibu Classic which I built up a bit. I installed a shift kit in it. I had two options "Firm" and "Hard". I was only 18 at that time, and all I was interested in doing was to preserve the life of the clutch packs, and of course chirp the tires when shifting. So, I picked the "Hard" option which involved removing accumulator springs.
It was fun for a while, but eventually all those hard shifts caused two failures:
1. Universal Joints (Not 100% sure if it was the hard shifts or not that did them in.)
2. Cracked flywheel. I had the flywheel replaced and modified the shift kit to "Firm."
My understanding of transmissions tells me that installing a shift kit to firm up the shift will actually extend the life of the clutches, since they do not slip as much when engaging. However, the trade off is more stress on the drive train. I heard of people blowing the rear end from excessively hard shifts.
As I previsously stated, my experience is based on a Chevy Turbo 350 transmission back in the late 80's. There probably are other people on this forum that can give you their experience in this matter.
Currently I have over 200K on my existing transmission in my Explorer. It still works perfectly. If I had to rebuild it, I would probably go for the "Firm" shift kit, or simply just replace the pressure regulator spring as other people on this site have done to increase line pressure, therby firming up the shift points.
