Rear end shimmy question

[dracula4999]

I have a 70 short stepside. I haven't had the truck very long and am still learning all the things that have been done to it. It has been lowered with blocks in the rear for sure but other than that I have no clue what has been done to the suspension. While driving normally, maybe quarter to half throttle, every time the truck shifts the rear of the truck seems to wiggle a bit. It feels as though the springs are really soft, or something is loose. I have looked under the truck and don't see anything out of the ordinary. Is this normal with a coil spring truck? My 71 gmc was leaf spring and long bed and didn't have this issue. Has a 350/350, feels as though it has a little shift kit. Any insight would be greatly appreciated.