First of all, front diff or rear diff? If rear diff, 2WD or 4WD? The Dana 44a is very uncommon in inline-6 Grand Cherokees, only found in *some* of the I6s with tow package, and I6 models with QuadraDrive (or the very uncommon SelecTrac w/ special-order rear LSD).
The aluminum Dana 44 in the WJs aren't awful but they do tend to have some case flex issues that can lead to bearing failure and/or abnormal R&P wear.
$1800 is a LOT of money for a vehicle that is only worth $6k on a good day. I would go used. www.car-part.com
to find an axle. Also, be sure to visit us over in the WJ forum, http://www.jeepforum.com/forum/f310/
- we can help you with DIY repairs, maintenance, modifications.