"Chris 4x4" says:The leaf springs on the Tacomas are on the soft side of the spectrum. That being said ( please follow me on this) When power is applied to the tires, the torque moves your rear axel up a little bit causing the springs to twist like an "S" just not as dramatic. When you are at crusing speed the springs relax into their normal state. Now, when comming to a stop ( with an auto and below about 15 m.p.h. ) your engine is still trying to turn the tires and the differential is moveing up a little bit twisting the springs. When you are stopped and let off the brakes or ease off them, the springs unload and you hear the "thump". A small amount a axel wrap is not a bad thing and is normal on most Tacomas. It can be tammed with the addition of an AAL. Blocks will amplify the problem and as they move the fulcrume point where the axel mounts on the springs, thus leading to premature wear on the springs. In many cases if the block is poorly made, they can shatter, crack, or whatever and cause a big problem.
<<<this post in another forum from a very WISE Man on technical stuff-"Chris 4x4"..........this is your answer-no cause for great concearn