The rear shock on a wing is marginal when new, and wears out quickly. You have over 20K miles on your bike, so your shock is pretty likely trash. The wham-slam over the bumps that your wife experiences is directly related to a lack of proper damping. Your preload adjustor is probably a bit low on fluid, and you aren't getting the full preload effect. Put the bike on the center stand and run the preload to zero. Start running it up, and listen for the pitch change. (It will go from eeeeeeee to oooooooo) Release the switch, and that is where your preload starts to work.
You didn't mention how much you weigh, or how much luggage you carry but the stock rear spring is way below what is needed. I think it's a .7 or .8, and a 1.2, or perhaps a 1.1 with a 115 lb wife, is about right for this bike, unless you are very light or very heavy.
So, between a sacked out shock, not enough spring to start with and the preload adjustor not able to crank it up, your wife is going to get slammed. The reason it feels worse to her is that she is sitting directly over, even somewhat behind the shock, so everything gets amplified for her.
Now you can try tire pressures, car tires, speeds, or sprinkle pixie dust on it, but if your wife is getting beaten up back there, your shock is the culprit. Any new shock, even a trike take-off will help, but a Traxxion or a Racetech with a 1.1 or more likely 1.2 spring will solve the issue.
=Dave=
olddogmoto.com