Honda Repair: shaking car..., strong vibrations, honda accord ex

Question
My '92 Honda Accord EX runs very well for the most part.  However, twice in the past month or so, it has started shaking somewhat violently while I have been on the highway going approximately 65-70 mph.  Both times it happened I had been driving for about 10-15 minutes.  The car started convulsing almost, and it felt to me like it was going to die (though it didn't).  It was hard to tell where the shaking was coming from.  It is a different sensation than a flat tire, or a bumpy road.  The steering wheel shakes, as does the car as a whole.  Both times when the shaking started, I couldn't really accelerate past 60 mph -- when I stepped on the gas, it wouldn't really pick up speed.  The shaking got slightly better when I got off the highway and slowed to a lower speed.

Both times, I was a few miles from my destination.  After arriving at my destination, I turned the car off.  When I started it up again (the first time), it was fine.  I don't know how the car will run this second time, because I was on my way to the airport (and am returning tomorrow night...so we'll see how the car runs then).

I had my car looked at by a mechanic after the first incident, and he replaced my exhaust pipe and muffler, but other than that, he said the car was fine.  The second shaking incident (on my way to the airport) occurred the day after I had the car worked on.

My question is, what is causing the shaking?  And is it dangerous?

Any insight would be greatly appreciated.

Thanks!

Answer
I would have to say it sounds like unbalanced tires or bad axles. If the tires were unbalanced they vibrate at high speeds but it would also make the alignment off sometimes so if it is that then re-balance tires. On the axles they can have a cracked halfshaft which causes strong vibrations while driving and sometimes will shake really bad or not depends how they feel really its kind of weird but thats how they work so i would really think that it sounds more like the axles so check the axles for any cracks if it is replace quickly or the axle will just snap while driving and that is very dangerous.  For the tires being unbalanced its not to dangerous but i prefer replacing as soon as possible.