Those who tow or intend to should read the tow rating descriptions on the Reese site at
http://www.reese-hitches.com/learnin...towing-classes.
The most important words they use are "up to" for load capacity. Any class hitch can be load rated at LESS than design capacity due to vehicle frame design or axle capacity. I had a Ford F350 DRW with a class 4 hitch, but it was labeled at less than published class 4 limits by Ford, for reasons I never investigated. With a weight distributing hitch, it had a capacity well above what I needed to tow a 3326.
Also, there are weight carrying receivers, and there are weight distributing capable receivers. Frame design or mounting point strengths for a particular vehicle may make a specific receiver unusable for weight distribution, since the stress points are quite different when you load up a WDH.