vibration at 25-40 mph

What would cause a mild vibration at relatively low speeds? I recently replaced my stock 7 1/4 rear with an 8 1/4 sure-grip and I installed the driveshaft from the 8 1/4 donor car because it was the right length already. Since then I notice a mild vibration starting at about 25 mph. It continues up to about 40 mph and after that it's not very noticeable, either because it goes away or it's just too mild to notice over the general bumpiness in the road. In addition to the rear end and the driveshaft, I also put in brand new u-joints, brake drums, leaf springs, and rear shocks as well as new wheels and tires (the wheels aren't actually new but they were balanced at a shop I trust). The only thing I changed in the front is the ride height which had to be adjusted to match the new springs. I realize with so many new pieces there are a lot of possibilities, but I thought the fact that it happens at such low speeds might narrow down the search.