Okay good info.
I'm going to poke a bit further here --
Q: What's truly driving your desire to look into installing a rear sway bar --- ?
Is it based on actual dissatisfaction with the way your van drives for you....or is it based on a mechanic recommending one to you....due to an opinion he has that you might need one?
I'm not sure about the understeer you're mentioning (which is usually the most desirable/neutral/safe handling characteristic to have), but a lot of the E-series vans are noted to exhibit potentially-unstable "wagging tail" behavior at speed on the highway, if a certain number of conditions are present. (Rear end swaying around.)
A rear sway bar *can* help with this.
Also of possible aid:
* stiffer rear springs
* better rear shocks
* a quality tire, often with stiffer sidewalls and more linear tread design)
Additionally, if your van has a not-uncommon-for-the-E-series narrow rear axle, where the track width is less than the front track width....then the installation of a **quality** set of rear wheel spacers can improve stability substantially. (Widens the rear track to match the front.....it becomes more resistant to swaying/lateral load-transfer.)
Paying careful attention to tire pressures front/rear is important too, they can have a huge impact on handling/stability.
Lastly there are steering stabilizers which can absolutely help improve highway stability in many instances, but which are viewed by many as a "band aid" fix versus an actual remedy to steering issues. It is of use to note that Ford actually installed OEM factory steering stabilizers on the E-450 and larger commercial versions of Econoline chassis vehicles, so the engineers in Detroit also clearly agree as to their application-specific value on heavily-loaded versions of this platform.
just saw that Carringb already chimed in --- he's the man.
His opinion is one of the most-considered and experience-backed on the forum.