Shaky Axle Over Bumps :/ ??

Hey guys so i have a 2011 Taco TRD with a lift toy lift and it has 285/75 tires bf goodrich with Ivan stewart 16rims. My taco has 37k on it, i always keep up on doing all the maintenance on time. However im having this problem where when im going over a small bump or speed bump theres a vibration/ shaky feeling coming from the back. Is this normal? I have not noticed this earlier, but this has been going on for a while. I dont do heavy offroading, this only happens at small speeds AKA 35mps over bumps, should i be concerned?
Ive read other topics on here and cant quiet find one that fit my priblem to fix it.

Ive looked at everything under the truck and everything seems to be tight, not loose or anything. I have no idea what this could be. Delearship doesnt know what im reffering to and im not giving them my truck for a week so they can "test it"

Any ideas on what this could be, is this normal since the rear is so light.
Please share your expireince and input

 

still having this problem 43k miles on the truck now. havent dont any serious off roading or hit anything. problem comes and goes. im wondering if i should get new leafs and take the block lift out?

 

Bed bolts all tight?