DON'T START REPLACING PARTS!!! Do things right, scan your stored codes first and go from there. The speedo sounds like a SS problem, which sits on top of the rear diff and held in place with a 10 mm (IIRC) BOLT. Check the ABS cables where they plug into the back of the hub, Why would you take it to a Bowtie dealer?? I would usually say take it back to whoever worked on it but not in this case. That's like hiring a plumber to remove your appendix!! Just pick'in at ya.
Good Luck
NCH