Traction bars prevent the leaf springs from wrapping up improving traction.
Usually, one end is welded to the frame, the other bolted to the axle.
Another bar which goes side-to-side is a panhard (or track) bar that would keep the axle in line with the front wheel track.
My 2WD has neither, but I think aftermarket kits are available.

 

Ford only installed them on the 4wd model because 4 low is really the only time the rear axle will wrap bad enough to cause wheel hop. Not many folks are going to modify or use their 2wd Ex in a manner that produces wheel hop so most don't need a anti-wrap or traction bar on the 2wd Ex.

 

Can't seem to find a good pic of those bars.
Do they bolt on?

 

Part of the factory spring package. It permits some axle wrap then contacts the body to resist additional wrap.

 

I've yet to see an OEM slapper bar extend to the spring eye.


This is only a representation. The OEM slapper snubber (yellow) is not near as beefy. It also doesn't extend to the spring eye.

 

Worth noting the anti-roll bar and traction bar just like the MacMathews was asking about.