I installed an 8.8 in my own car, but a friend is having me narrow, link up a 9 inch for his. Narrowing, I bet its the same for both. Took me about an hour to narrow the 9 inch after deciding how much to take from each side.
The 8.8 comes with c clip axles, but with disc brakes I doubt theyd ever fall out, but I ended up installing the Super88 kit with c clip eliminators, 9 inch outer bearings and chromoly axles.
The axle tubes need welded to the center section, but with the cast/steel difference, its kinda tricky to get a good pen weld that wont crack out. I did a tq arm setup like the Fbody cars and tied the tq link into the diff cover to handle the power.
The 8.8 is a little ligher, and the center section is smaller. Depending on the width you need, the 31 spline Explorer axle comes in at about 59.5 inches with one axle being 3 inches longer, so you can narrow one side and use the stock short axle for a 56.5 width rearend.
Huge difference is the really low pinion on the 9 inch. Its awsome that is support with 3 bearing, but damn its hangin low. The debate is out there that it takes a little more power to turn the 9 inch over the 8.8.