I think this was cleared up already, but here is "my" attempt for the benefit of others and for myself if I got this wrong to be corrected. After a bunch of time trying to figure this out here is what I came up with.
This link
http://www.toyota-4runner.org/4th-gen-t4rs/94003-tow-hitch-2.html#post826896
shows the standard hitch "Reciever" found on the 4th Gen V6 4Runner. It bolts up to the middle of the last crossmember hidden by the plastic bumper cover rated for 5000lbs. On the V8 this is also rated for 5000lbs.
This next link is a hitch receiver that is similar in concept to the optional V8 hitch receiver. There is nothing stopping you from installing this type of receiver onto a V6 4Runner. Notice that it bolts to the frame rails on both sides and not just right in the middle. That is key here in its' ability to distribute weight. Alos note that there are 2 weight ratings.
Weight Carrying: 600/6,000 lbs. (TW/GTW)
Weight Distributing: 730/7,300 lbs. (TW/GTW)
So now if the truck that this gets bolted onto, is capable of pulling and stopping the load, it now has a "receiver hitch" "Rated" for 600/6,000 lbs. (TW/GTW) "weight carrying" meaning that the total weight of the load, trailer+whatever is on the trailer weighs max 6000lbs.
http://www.hiddenhitch.com/content/products.aspx?lvl=3&parentid=1000&catID=1040&part=70779
If you then add this contraption, you will be able to make use of the Weight Distributing: 730/7,300 lbs. (TW/GTW) "Rating" of the hitch receiver. Even on the V8, without this contraption you probably shouldn't be towing the 7000lbs. It is not just the receiver that dictates you capacity, but the whole weight distribution system.
Common Weight Distribution and Sway Control Questions | etrailer.com