tranny shutter

I have a '97 Venture, 170,000 kms..the transmission shutters when it hits overdrive, everytime, all the time, and only in overdrive. It's like driving over a rumble strip.
Other than that, the tranny shifts fine through all the gears. I have had the fluid changed, engine tuned, fuel filter changed, still happening. I assumed it would be the lock up converter, but the mechanic who checked it out was not so sure. What do you think, is this a common problem with these transmisions?.. Thanks

I'm not a tranny mechanic, but I do have suggestion that might help until someone more knowledgeable posts a reply. See if it happens if you drive in 3rd for awhile instead of "D" (which is perfectly fine to do long term -- it will just cost you a little gas mileage). By driving in 3rd, the van won't go into overdrive, but the converter will still lock up. That should tell you if your problem is with the overdrive or the converter.