[dsraven]

you may also want to check that the bushings are tight in their mounts. it wasn't uncommon for the pins and bushings to get so worn that the pin wore right through the bushing and started to oblong the mount casting. another point brought up by mr48, you can't grease these old trucks enough. the new bushings may have been pressed into the out of round hole so they have an opprotunity to rock in the mount as they "wear in" to their new mount.
curious, if you are using the stock narrow springs, did you remove any leafs to get your current ride height? if so, and you are seeing lateral axle movement, it could be your springs flexing. they are spring steel but steel doesnt know which way it is supposed to flex so it flexes in all directions, just that it flexes easier in the way you think of first when thinking of vehicle leaf springs. if the spring pack has been altered to be less tall, so less leaves bolted together, then it will be less rigid in all directions, not just up and down.
still, check out the movement by having some strong friends push you vehicle's rear side to side while you watch where the movement is. of course a video camera attached to the underside while you drive some tight curves would do the same thing, more spendy though.
you could attach a sharpy marker to the body or frame so that the felt tip rubs a piece of tape stuck to a tire and then do the body push from both sides and that would give an indication of how much it moves by leaving a mark on the tape. not that you need a dimension, just saying if you want a dimension to keep track of what you did to remedy the problem and what the outcome of that fix was.

first ensure the body is centered on the frame and the body mounts are tight
second check the spring pins and bushings and their frame mounts to ensure all is good, possibly do the shackle mod if you feel it may help, its not gonna hurt anything and is removeable if you find you don't want it
third ensure the spring u bolts are tight and the springs are sitting down nicely on the spring pads. often if there is movement here you will see a mettalic or rusty colored "dust" in the area. remember that the spring pack has a bolt holding it all together and helps locate the springs on the springpad but it is the U bolts that keep the springs located there and they also rely on a good flat smooth surface for the springs to sit down on and be tightened against. also, new U bolts should have been used upon install and they typically require a retorque after a short time of driving. if not retorqued then they will not be working at their intended strength. when a bolt is torqued up it becomes like a spring, it is stretched to it's elastic limit and so keeps things tight. overtorquing makes it go past that limit and may cause breakage, undertorquing can do the opposite and allow the nut to loosen. any fastener supplier will tell you that bolts are made to be used once as when they are torqued they are stretched and don't actually return to their original length. thats more important for stuff like suspension parts because a lot of those are tempered steel
fourth have your buddies help you do a body push/rock while you check visually to see if anything is moving. check the spring pack as well for lateral movement
fifth, if you need to get this far, check that the axle is centered in the frame. use a plumb bob and make marks on the garage floor. mark each side frame ouside edge, each side wheel mounting surface and each side spring center, then get out the tape measure and do your checks and comparisons. adjust as required
sixth, if need be, use a come along or a rope and pulley, whatever, set up from the frame on one side to the axle on the other, and physically pull the axle one way and then the other and see what is causing the movement. when the vehicle is driving and turns a corner there can be quite a bit of sideways forces so using a prybar on suspension parts to simulate that force may not be enough.
seventh, keep us posted on your outcomes
hope something in there helps you find the problem