I found out once again I'm never too old to learn something! When I had the rearend out of my gasser recently I reset the pinion angle with the heim joints when it came back from having new gears installed.
Until Sat. afternoon I had not driven it much over street speeds, around 35-45 mph. On the trip to the West side of town I jumped on the freeway and ran it out a bit, only to notice a vibration around 55 mph. On the way home, same thing again, but seemed even worse, so I kept it at 50 mph or less.
Today I decided to pull the ladder bars, so I got under the car and unbolted them. I noticed the pinion angle was a bit nose down from level like it used to be, so I adjusted the top joints out to bring it back to level.
After putting it all back together I took it for a drive and the vibration was gone! Amazing what a few degrees of angle can do to cause a vibration in the drivetrain!

 

Holy crap! I have an 83 Silverado with a locker rear (4:11 gears) and it does the same thing. Bad vibration around 55/60, 5-10 mph above or below and it goes away. But... it's lowered 4" and still perched on the leaf spring axle plates. How would i adjust that out? Ya think i have an angle problem?

 

For the least amount of vibration for normal street use, the pinion angle should match the tail shaft angle. In other words when the car is sitting on its wheels, an imaginary line through the center of the transmission output shaft should be parallel to the imaginary line through the center of the pinion gear. If the angles are not close the universal joints will bind and release with each rotation and give your butt a massage.
During hard acceleration, such as in drag racing, the nose of the differential tends to rotate upward and the two angles are no longer parallel, so if you have adjustable rear end control links it is possible to adjust the angle to give best results. Usually a few degrees down.

 

Lowered can also cause pinion angle issues. It can create a situation where the driveline and pinion are on different planes, and possibly give the vibration issue. It's actually pretty easy to fix on a leaf spring vehicle. They make wedges that go between the spring and the axle to tip the pinion up or down, depending on which way they are installed.
First thing you need to do is get a degree wheel that's magnetic and check both. Start by jacking the car up and supporting the rearend on jackstands. Then with the degree finder you can put it on the flat suface at the bottom of the pinion to see the angle. Then attach it to the driveline and check that angle. 5 degrees difference is a good point to be. A little either way is OK, but more than about 8 degrees difference will cause issues, and less than about 3 degrees will not lube the U joint properly.
But before doing any of that, be sure to have your wheels checked for trueness, and balance. If they're good, then take a look at pinion/driveline angles and adjust.
Harbor Freight has a nice little angle finder for under $5 that works perfect for this!
Dial Gauge Angle Finder

 

Ahhhh... Thanks guys! I've done a dozen engine and rear end swaps over the years and i thought my Silverado had a ring/pinion issue or it was the locker diff. I have 2 sets of wheel tire combos and happens exactly the same with both so i know it's drivetrain. Guess i'll be shimmin' soon! Thanks again!!