Paul, been towing with a 3.0 CRD Grand Cherokee for over 2 years , van is a 1700kg Delta twin axle. GC has no problems with the van and certainly have never had a problem with it looking for gears. The auto box fitted is recognised in the industry as one of the finest boxes available. As SPG says only thing is a bit to much plastic , but there are plenty of suppliers in the US where trim and extras can be siought.
As for the 2.7 discovery, we had one as a loan car when an idiot hit us, it is a nice vehicle but engine and gearbox wise no match to the Grand Cherokee, a lot slower of the mark (less torque and HP) and the engine is a lot nosier than the Grand Cherokee, which is almost silent, even at speed.
Go to your local dealer and give one a test drive.