Dave C, the '90-'94 Pantera Si did in fact use an electronic version of our old mechanical speedo. Not sure, but I think the same ADP drive had a 2 wire electronic adapter screwed on where the cable drive goes. Then the wires terminated at the speedometer into another small electronic gizmo. But I wonder how that expensive bolt-on system gravitated onto an early Pantera? I passed up one from the Si crash test car 20 years ago because I couldn't afford it.
If an electronic sender is indeed broken, I cannot help. Perhaps Jon Haas could. He would probably need the sender on the speedo end for comparison. But if your system is entirely mechanical as was stock, I've repaired a dozen of those. Look closely at whats on the ZF and take some photos, please.