Starter took a crap tonight

[ShinyC10]

Driving home from the gym tonight, going about 35 and all the sudden the starter begins to make a horrible noise. I slowed down to listen and the truck was still running but my starter was constitutionally cranking as if I was holding the key to try and start the truck. I pulled over and turned the truck off and pulled the key out but it continued to crank till my battery died and I was stuck on the road. Pushed it a couple blocks home and under the car port. Any idea what it could be? The starter ain't but 3 years old tops. This is in a 1986 Chevy pickup with a 350 and 700r tranny. Thanks in advance