Knocking under truck when hitting bumps/ 1997 taco bumper on 1998???

there is a knocking sounds coming from under my truck when the back tires go through a bumps. it sounds like its damped or surrounded by rubber or something. passengers say they can feel it under their feet but i cannot think of or see a thing that could be doing it. it started after i took it to get a new transfercase seal put in.

this parts weird it really gets amphified in the wet. i am confused as hell as to what this could be. thanx.