The tubular Jeep 4.0L manifolds (were replaced w/cast iron in 1999) crack because of the weight of the exhaust system along with flexing due to heat/cool cycles. The easy preventative is to install an exhaust hanger right at the firewall, or as close to the front as you can. That takes a LOT of stress off the system and will prevent most cracks.
If one does crack, get it welded instead of replaced. Ask the welding shop to strengthen the cracked areas with gussets. I have my 87 exhaust welded and even welded a support bar about halway down and between the rear tubes along with some gussets. No cracks since, and it was welded four years ago. I'm not using a front hanger either.
