I have an FG, which is pretty much the same rear end. The rear end wiggle or "wobble" (feels like you're caught in the draft of a semi) is caused by toe changes, and is usually worn (or stuffed) control blade bushes. These are mounted under the rear seat, at the front of the control blades. When these bushes are stuffed, the toe angle changes all over the shop, which rapidly increases tyre wear.

In addition to those blades, there's 3 control arms on each side - a cast (boomerang) upper one, a pressed front upper (U-shape in profile) and the lower control arm with the spring seat on it.

In those 3 arms, there's a combination of bushes, and what are effectively rose joints / heim joints, or what Ford calls "Cross Axis Ball Joints" When these CABJs wear, they rattle, so if you don't have rattles, they will be fine. However, the rubber bushes do wear over time, and get sloppy. The inner bushes on the lower control arms affect the camber, but they won't have that much of an effect on tyre wear or stability. However, the upper ones, particularly the front, can also affect the toe angle, so check these out.

Without knowing what your mechanic replaced, and the fact you still have symptoms, I'd be thinking he changed the upper control arms, but not the control blade bushes.