MY 740 SE WAGON HAS A TROUBLING VIBRATION THAT SOUNDS LIKE IT'S COMING FROM THE REAR-END. IT SEEMS TO HAPPEN MOST FREQUENTLY AFTER A LONG DRIVE.

DOES ANY ONE HAVE AN IDEA WHAT THIS COULD BE?

I would guess your rear U-joint is seizing when it gets hot.You can either block the front wheels with the car in neutral, jack up the rear to spin the drive shaft, or simply unbold the U-joint from the rear end.

My car had this problem, causing the pinion seal to leak. Check the hanger bearing on the shaft while you're under there. If everything doesn't spin right, you'll have more wear from the tranny to the rear end. Some mechanics will recommend dropping the shaft and replacing all joints and hanger bearing at the same time. Depending on mileage, you'll be doing it at some point anyway.

The heading of your posting provided the best clue. It sounds like the rear u-joint is failing. Do you feel vibration when coming to a stop as well?

a little, but it is most severe when i take off

With IRS, vibration on take off is commonly the result of a misaligned drive shaft. I'm not sure if it's prevalent with solid rear-ends but it may be worth a look. The fix takes less than an hour.

I've attached the bulletin Volvo issued and if you do decide to move the center support bearing, I suggest you go with the bulletin's recommendation to first try the upward adjustment. That seems to have worked for other Brickboarders.