Depending on where the dent is, you may be able to do a couple of things to mitigate the vibration. (which may or may not be caused by the dent) First thing to do is mark the shaft at both ends in reference to the flanges. Then swap the shaft end for end. If you have a ball and trunion u joint on both ends some of the vibration may be coming from either or both of them. Flipping the shaft forces them to work in a different spot in the trunion. Sometimes the trunion will get worn in the areas where the balls rest most consistantly, and glipping it will alter that relationship. You put the reference marks on so the shaft will go back on is the same rotational position fromwhichit was removed. If this doesn't work, try then rotating one end 180 degrees in relation to its flange. Probably most effective at the tranny end. Just drop the retaining bolts, turn the shaft 1/2 turn and reconnect. This should put the reference mark 180 degress off on that end. If that doesn't work, then do the same to the differential end. Of course this will require a road test between ieach step, but it may lead to a solution to your at speed vibration. Some one also suggested you can use a strobe light, like a timing light, flash it on the drive shaft with the car runing and the rear wheels off the ground probablyin second gear. The strobe will showuo if it is spinning truely or wobbling from the dent. You can also use hose clamps around the shaft to attempt to balance the shaft. The mass of the tightening screw works like a tire weight to damp rotational imbalance. try all these before looking for a driveshaft shop. The drive shaft shop wopuld most likely be foundin your Yellowpages under drive shafts or U joints. And is probably located in that seedy industrial side of town where they fix commercial equipment and trucks.