I had a vibration on my 7th Gen accord when I purchased it.
- It turned out to be a slightly buckled fornt wheel. (which I still have to replace as its on the rear at the moment)
But the diesel accord does suffer from weak drive shafts (loads of threads on this)
As mentioned about check the wheels are properly balanced. I mean properly as some garages don't bother doing it properly
Also check to see if any bulges on the tyres
Jack up each corner and spin the wheel and ensure both rim and tyre are round with no lumps etc
With the harshness of the ride I feel the accord is a very comfortable car to drive.
Is the suspension upgraded? Maybe it has the factory A-spec lowering kit which will be little more stiffer
Tyres can play a big part with ride comfort. What tyres do you have on there?
Check tyre reviews.com to see how your tyre brand rates for comfort
Ensure the tyre size is correct (i.e. for 17s it should be 225/45/r17)
Correct tyres pressures (there should be a sticker on the inside of the door showing you the correct pressures to run at or give you a guide according to your situation)
Hope it helps - just thoughts from the top of my head