Aerostar speedo problem

'93 XL, 3.0. This rig is amazing. At 230K, it still has great oil pressure, plenty of power and does everything asked of it. I had it
into the shop to replace the ignition switch, the idiot oil pressure sensor, which reads much lower than reality, and the headlight switch. When I got it back, the speedometer, which was always smooth and accurate before, started acting erratically. Huh? It would "bounce" just a tiny bit at 30 MPH+. At 50-70 MPH, the needle bounces wildly back and forth (+/- 20 MPH), only occasionally hesitating... just long enough to have some idea how fast I'm going. I could ignore that and go with the flow, but it also rendered the cruise useless, which really sucks. I just drove a 1400 mile trip using my now, very tired right foot on the pedal.
Being old school, first thought was that the mechanic somehow crimped the speedo cable, while behind the dash. But alas, I found that this rig doesn't have a speedo cable. The now-off-the-hook mechanic suggests that it's either the speed sensor, or the dash unit itself. Before I start randomly replacing pieces, I'm sure some of you have experienced this and can venture an educated guess.
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