I have a 2004 dakota 3.7l 4X4. In the owners manual it states that the tank capacity is 20 gals w/ORVR and 22 gals w/o ORVR. what is ORVR?

From
http://www.stationwatch.com/swfaq/orvr_faq.htm

The initials ORVR stand for Onboard Refueling Vapor Recovery.

The concept of ORVR is to include a system within an automobile (or similar automotive vehicle) such that vapors accumulated in the fuel tanks, as well as new fuel vapors generated during refueling, are captured within the vehicle ("onboard") rather than depending on external methods (known as Stage II vapor recovery) to efficiently collect the vapors before they can escape into the atmosphere.

Current ORVR technologies generally consist of Activated Carbon Canisters (ACC) which absorb the hydrocarbons (HCs) contained in the vapors forced out of the vehicle while refueling is occurring.

steve98312, now that you have explained what it is, how do I find out which I have. According to the website I assume that I do have ORVR on a 2004.

You most likely do have it, but to be sure, I suggest a trip to your local Dodge Service Department.
They will be able to tell you for sure. Have your VIN handy, or maybe a service tech can just look under the vehicle and tell.
Sorry I couldn't be more help.