I'm looking at buying a used 2002 Chevy Express 1500. It looks like it has an aftermarket hitch on it that is rated at 6000lbs. Does the van need anything else to be able to tow that much? Does the factory tow package change the gearing at all?

 

My understanding that on the 1/2 ton models, the towing package only consists of the platform hitch and plug. The tranny cooler only comes on 3/4 and 1 ton models with the towing package.
There is no gear change. You either have 3.42 or 3.73 gears.....the latter is better for towing.
6000 lbs may be the hitch capacity but its more than that van can tow.
I had a 2002 3/4 ton express with the towing package, and the vortec 350 and 3.73 gears and it was rated at 5800 lbs.
check also for your vans gross combined vehicle weight. That is trailer plus cargo plus van..... max overall weight. That van is prob around 10000 lbs....