The car has no u-joints. It has only CV joints on the front half shafts.
Unfortunately these minor & intermittent problems can just be really hard to track down. Is it safe to assume that the check engine light doesn’t come on?
When it is vibrating at idle are you thinking that it is misfiring? Or might one or more of the mounts be broken or deteriorated? If you just pop the hood & watch is there a lot of engine vibration?
Have you checked the fuel pressure? If it is borderline that could give rough idle and the occasional hiccup while driving.
I’d also check to make sure that the intake manifold isn’t leaking. You can use a propane torch w/ a hose rigged up to run propane around the intake while it idles. Or you can spray something like carb cleaner around the seams. If you hit any spots that are leaking the idle should respond.
There are a couple of things that could give an intermittent vacuum leak. One would be a sticky EGR valve & another a sticky evaporation purge valve. There are times that they are supposed to open but if they got hung up once in a while the computer wouldn’t be adjusting the fuel mix. Those are a long shot, but what they heck. For diagnostic purposes each of those things could be capped off to see whether it makes any difference.
Let us know how it goes.