What you are looking for are "Leaf Spring Shims" or "Axle shims" which go between your leaf springs and the perch on your axle. The shim is like a thin cheese wedge which will change the angle of your pinion in relation to the drive shaft. You can angle your pinion up or down just by turning the shim around. Axle shims come in different degrees and widths. most common are 2, 4, 6 degree and vary in widths from 2" to 3". First to determine which one you'll need, you have to measure how wide your rear leaf springs are. My guess would be around 2.5". Then you need to figure out what degree shim you'll need. This is the tough part because you'll have to expirement a bit to until the driveline is angled properly and the vibrations are gone. Since shims are relatively cheap, you could try starting with a 4 degree shim to see if you'll need more or less depending on the severity of the angle. The hard part is that you'll have to remove the center pins from your leaf springs (while compressed) and replace it by sliding the pin through the wedge, attatching it to the leaf spring pack. If you do not know how to take apart your leaf packs then you should consider taking your truck to a shop and have your driveline angles corrected by them.
Keep in mind the that the closer you get both angles on each end of the driveshaft, the less chance you'll have vibration problems (when accelerating).
Here's a site about axle shims!