In my experience death wobble comes from either excess movement of the track bar (worn bushings or ovalized bolt holes) or worn ball joints. Since you already replaced the track bar (and, while doing so, presumably inspected the bolt holes), I would look at the ball joints and replace if they are worn to the extend of up and down motion. Sounds like your mechanic recommends replacing your tie rod end at the pitman arm. Those are cheap and should be replaced if worn. As for the pitman arm, I’m not a fan of drop arms, but unless there’s something wrong with yours, why replace?