91-92 4.0 Wranger vs. 93-newer

Can someone explain why some after market parts for the 4.0 do not include
the 91 and 92 model years? I have a 92 that has the typical leakey valve
cover. I decided to replace it with an aluminum cover, but I found most
catalogs only offer covers for 93-04. After looking at other items there
were quite a few that also skip the 91-92 years. Some of those parts
include cap and rotors, spark plug wires, and headers. The thing that I
find really odd is that I have found some brands that say the same parts
listed above work for all years 91-04. If there are no differences, why
would these manufactures and resellers give up sales for those two years?
So, what are the differences between the 91-92 and the 93-04 4.0 engines?
Thanks!