Drive Line Shimmy in '97 S-10

I am having problems w/ my '97 S-10 x-cab. It has a 4cyl, 5 sp. manual transmission and is giving drive line (or what I think is drive-line) shimmy/vibration on acceleration. I really notice it when shifting between

1-2 and 2-3...at higher speeds it seems to even out. I had the truck in for service and the mechanic checked the drive shaft and replaced the clamps and checked the yoke for wear- all okay...still shimmies and vibrates. Any ideas???