The two dissimilar metals cause a host of problems, unequal thermal expansion, different hardnesses cause surface inconsistencies, different stiffnesses cause unequal deformation to torque, etc. The manufacturer saved a couple of $$ on aluminum. Under normal operating conditions (highway) it's a good/cheap decision. Most people with ZJ's don't really pound them hard anyways.

91 YJ
2 Seater Sand Rail
Handfull of other fun toys...
And a brand new, spit shined, B.S.
Oh, and a job to boot.