Replaced starter but it engaged while driving now it's fried

Hello,

1994 Ford Taurus 3.8. Wouldn't start, no click or anything so I checked battery/cables/terminals - all good and plenty of juice. Put test light to positive line to starter and turned ignition. Lighted turned on and off as I turned the ignition on and off so I didn't think it was the ignition. Replaced starter. Started right up and I drove it around a mile or so. Everything A-okay.

Next day driving down freeway and heard a grinding sound, like an axe on a grinding wheel. Smelled something burning. Tried turning car off but it wouldn't turn off and starter wouldn't disengage. Starter finally quit as it fried, but burning smell continued. Disconnected positive battery cable to stop the car. I believe I wired the new starter correctly as options are limited by post size and ground wire length. Tempted to try another starter but before sinking another $128 into it I'm curious if maybe it is something other than the starter. Any ideas or suggestions?

Thanks!