You will get an OEM part from places like GMParts.com, GMpartsdirect.com, GMPartshouse.com. These places are distributors or resellers of genuine GM parts. Most have no physical inventory. They take your order then submit it to a GM warehouse for fufillment. The part is then shipped from the warehouse either to you or to the reseller to ship to you.

 

gm part# 25854903

 

I believe that those vendors are actually GM dealerships who have set up internet parts stores. They only sell genuine GM parts. I've purchased from the second one several times and they have good service.

 

I think what was trying to be established in OEM vs. Aftermarket was that you will not be able to find an Aftermarket or non-GM brand tow hitch that will look like the factory OEM tow package.
An Aftermarket will mount below the bumper and its brackets would extend upward to attach to the chassis. This will affect the departure angle of the vehicle. Not by much but the variance will be less than the stock specs. If you frequently go rock crawling or otherwise use your vehicle where you would want the maximum possible amount of approach/departure angles, then you may want to take this into consideration. Of course, if you go rock crawling you are also putting some vehicle components at risk that are worth far more then the $120- aftermarket trailer hitch so it may be a non issue.
The factory (OEM) tow package mounts behind the bumper with only the receiver sticking out of the cut out opening that is made in the bumper. As stated above, it also has provisions for attaching the 7 blade RV electrical connector whereas the Aftermarket will not.
Although an Aftermarket setup will function the same as the factory OEM setup, (check the Aftermarket hitch ratings to be sure you can tow the same amount as the vehicle recommendations indicate) it will not look the same. The OEM set up has been tested by GM to determine that vehicles towing capacity and has a much cleaner look and is aesthetically more pleasing to some. It also costs a hell of a lot more.
The GMPartsDirect website I listed above is helpful as it will list all the components needed/required for the factory OEM tow package. As with most Genuine GM OEM parts, they are much more expensive than its Aftermarket counterparts. (Also, see Bunger's post #5 above about contacting Ridgedemon.com for OEM takeoffs from other H3's. You may be able to acheive the same factory result without the factory OEM prices.) The Aftermarket is much less involved to purchase, install, and does not brand the GM logo nor have that "factory" look and thus cheaper as a result.
If you have the extra cash and want that factory look, then go with OEM. If you want to save some cash, don't mind how it looks, and don't mind doing your own wiring, then go with Aftermarket. They'll both perform the same way no matter which you choose.
Hope this clears up some discrepancies.