um, because one random day driving to work my 35 started spitting out ring gear teeth? Although they aren't made of cardboard, they get that "weak" reputation for a reason.
The 87 4.0 manual trans is the dreaded Peugeot. If it works fine now, I'd keep it. But I'd also start hunting for an AX-15 for later down the road. There are a ton of threads on transmission swapping in the link in my signature.
SOA gives around 6" of lift and that can be quite expensive to match up front which might be why some people choose a smaller lift amount.