With an SYE, your u-joint angle on the axle end must be zero degrees, or just under zero degrees. This means that the driveshaft and pinion should be perfectly parallel.
Here is why (a synopsis of the above article). A universal joint running at zero degrees rotates in a perfect circle. As you introduce angle to the joint, the arc of travel becomes an ellipse, which means the assembly slows down and speeds up twice per revolution. In a standard driveshaft with a u-joint on both ends, the angles have to be very near equal (and opposite) so the arc of travel of both joints is equal. Otherwise the u-joints will be "out of phase" and you will get a harmonic vibration. This is a droning sound, that "cycles". An out of balance shaft will just vibrate, it won't make the cyclical droning sound.
A double cardan shaft, which we often refer to incorrectly as a CV (constant velocity) shaft, has two u-joints on the t-case end. The purpose is that these joints cancel out each other's angle. This means that the single joint on the axle end must travel in a circle, or it will be out of phase with the double cardan joint. Hence the zero degrees.
Back to the original question - a longer shacle, boomerang or not, raises the pinion by lowering the axle end spring mount (it turns the leaf mounting surface up). If removing the shacke cured the vibes, then you need to lower your pinion back to zero degrees with a shim so the u-joint angle as measured from the cap is -1 to 0 degrees in relation to the driveshaft. Rusty's leafs assume a SYE, which is why they point the pinion up. Along with a shackle, you should be close to parallel without a shim.
Hope this helps.
Nay