Well I run a 21 spline input on the rear case with the 4.7 gears. I have read that you can snap the input running around with just the crawl box in low range. It happened to my buddy Andy in his IFS 4Runner. He has since gotten a new 4.7 input gear and had it cryogenic treated.
Big thing is don't run the front case in low and the rear in high and get bound up, things will break. Typically I run the front case in low on the access roads and blues since I am moving at a decent speed and not getting bound up. Once things need to slow down, shift the front case to high and the rear case to low. If too slow, upshift the transmission, if not slow enough, put the front case in low.
I also tend to not run in first gear since it pops out, so I run in second and up. If I need to go that slow, I have a bungee cord that holds it in place. Ultimate low is generally to low and I end up up shifting to get some wheel speed, not easy to do when the transmission shifter is essentially locked into first gear by the bungee.
To my knowledge, no one makes a chromo front input, just 21 versus 23 spline. So far I, along with a number of others haven't had any issues. Anything can break though.
