Ranger starting problems

My dad has a 1988 6 cyl 4x4 Ranger he plans on giving me. When we try to start it it grinds. Sometimes it won't start at all. The starter and flywheel were replaced but after a while it started grinding again. If we roll it down a hill it will usually jumpstart. He said when it is running it drives fine. I don't really know anything about diagnosing but the only other suggestion we got was the main bearings are worn out and when it's started it might be pushing the engine up or something. I really don't know but if this is something that can be fixed without major work then I'll take the truck.