I agree that U-joints could be the issue. I would also think about checking, or having checked:
1.) slop (otherwise known as excessive lash) in the pinion/ring gear. check front and rear
2.) Wheel bearings/hubs
3.) front cv shafts binding and unbinding.
4.) grease the slip yokes (front and rear) as discussed in a sticky in the "under the hood" forum
5.) Also, hear's an interesting one that I think frequently gets overlooked; check the three rubber bushings that hold your front diff to the chassis. If they get worn, the whole front diff assmbly shifts around, and screws w/ front driveshaft and cv shaft angle. bad motor and/or trans mounts could have a similar effect
These are just some suggestions based on your somewhat limited description of the problem. Also, I'm assuming by "giving it gas" your referring to a quick burst of rapid accleration that torques or loads the chassis and/or offending part. If you actually mean that you're simply accelerating to higher speed, and that constant higher speed makes the problem go away, then I would also look at tire balance. Despite what the guy at the tire store might tell you, that 1/4 oz of imbalance can matter.
:scratch:It might be helpful if you could give a better description of the problem. Is this "roughness" only while driving? Does it run smooth at idle, in park, in drive? Do you feel it in the steering wheel (usually means it's in the front of the vehicle), or do you feel it in your, ahem "seat". This usually indicates the problem is towards the rear of the vehicle.
