Take a look at your rear leaf spring bushings for cracking and make sure the bolts are centered in the bushings.
Look that the leaf spring center pins are still centered over the spring pad as well as between the 2 stacked blocks.
Look at the condition of your rear spring U-bolts and their tightness, you should be able to see if they have been moving around at all.
One of these things probably isn't up to par.
You can also check that your rear wheel bearings are still good by jacking the rear diff off the ground and give each wheel an up and down wiggle or shake with your hands. There shouldn't be any clunking or movement really.
Double stacking blocks isn't the best idea but many kits reuse the factory blocks and add another block on top of them. For some trucks the factory blocks also double as the bump stop pad and need to be reused. If they are both steel I don't see why you couldn't weld them together.
The sway bar removed won't cause what your saying.
Rear links don't mix well with rear leafs tho, as both move up and down in a different arc and because of this, if combined, they will fight articulation.