The Jetta has a solid beam in the rear. Does have Koni Red's for struts/shocks, albeit set to soft setting. Pretty stock otherwise, and with narrow rubber (195's).
I did go back yesterday, with the Jetta, and at the same speeds--25-30mph, give or take--the car was basically fine. Only in one spot did I think it was getting really loose, and in that spot it was clear to me that I could easily do so with a flick of the wrist. Actually, it was sliding around a bit, just not that badly. That stretch of road is signed at 30 or 35, I forget, and is usually paved, and gets a fair amount of traffic. [For some reason the town decided to strip it just prior to winter.] So I guess the truck is being just a truck, and I'm driving too hard for the road surface.
I do have to admit, I'm realizing now that I did pump up the tires on the truck, a bit over the door plate. I did some reading on "best" pressure for the tires I have, and it seemed like 36psi (rather than the 30/33 on the plate) might have best tire life. That no doubt is hurting, I forgot all about that.