The blazer is a 2000, 124,000 miles, automatic transmission. This problem ONLY occurs when I'm going around 50mph and accelerate lightly, the whole thing will vibrate kind of like it's shifting into too high of a gear. If I give it a lot of gas, then there is no problem/let off the gas, there is no problem. Also, this is more prominent after driving for a long period of time. Besides this, there are no other problems with the transmission that I know of. I just had the u-joints checked out and they are in good shape.
I hoping that it's something besides the transmission. If it is, I hope that it's minor. I'd like to hear anybody's ideas on this. Thanks.

 

wheel/tire balance?

Is the suspension stock?

 

Almost sounds like torque converter shudder. How does your transmission fluid look?

If you press on the brake while you are experiencing this shudder, does it go away? You only want to press on the brake pedal enough to light the brake lights, but not really apply the brakes.

 

Motor/ tranny mounts?