Diagnosing starter problem - jeep wrangler?

Hello, I have a 98 Jeep Wrangler that broke down yesterday. I parked the car and then when attempting to start it two hours later, I heard a grinding sound. My brake light and air bag lights were on but I couldn't use the radio/heater etc. The car drove but the sound got worse a few miles down the road. I realized that my starter was starting to smoke and I knew that the pinion on the starter wasn't disengaging because the flywheel was turning it even after the key was removed from the ignition. This freaked me out!! lol. I was able to replace the starter and it ran fine but there was one time that the starter made a similar sound as the old one when I fired her up. At this point, I'm not sure what the problem is but I don't want to ruin another new starter. Poor battery connection? flywheel problem?? Thanks for any input.