There are five terminals at DFW and all five are connected either by SkyLInk, or by Terminal Link.
SkyLink is a tram that runs above the gates. There are two entry points at each terminal with elevator, stairs, and escalator access. The trains run every two minutes in both directions and you should be able to get from one terminal to the other in five-ten minutes via train, but be sure to allow walking time to get to the SkyLink and to the gate.
You can clear security at Terminal D and go to any other terminal from there. However, if you are flying Jet Blue and checking bags, and your bags either are too big, or have liquids of greater than 3 oz. (even one item greater than 3 oz.) then you can't take that bag through security.
In this case, you would need to use the Terminal Link bus. The buses run every ten minutes and there are also two locations at each terminal to jump on and off the bus.
Neither of these options costs money or require you to show your tickets to use them.
Edited: 13 February 2012, 8:09