Flat Nasty has got plenty of trails built for lots of different rigs. The hard trails are bad to the bone with no-bypasses. Interm. are great trail riding fun with challenges. The easy trail aren't too hard but a challenge for the stock rigs.
The CJ a 87" WB with a 4.0 EFI, AW4 and factory short shaft D300. 3" stretch to the back, front pretty much stock location. The rear shaft is 16.5 at level. If you running a 4.0/TH350 or 999 with the D300 it be about 2" to 3" longer approx. You got to watch the D-train postion in the chassis. We sunk it as low as possible motor and tranny with a fully clocked D300 for better CG. The t-case is at the lowest point, including skid, ~1" below frame rails.
If you were to choose a 4.3 or a 350GM you have even more lenght. We choose the make a 98' HO from a ZJ, think its still lives there. What it doesn't know is it's in a 1980 CJ5.
-John