87 Ranger - Starter gets stucks Engaged?

[Preveda]

Hi, I am new to this site and tried doing a search but I couldn't find any problems quite like mine.

I have an '87 Ranger, 2.9L (Gas, FI), V6, 2dr.

When I go to start the truck the starter goes to engage the engine but it seems to get 'stuck'. It makes a horrible grinding sound that won't stop. Even with the key out of the ignition it continues. Disconnecting the solenoid's grounding cable doesn't seem to shut it off. Sometimes slamming the hood stops it, sometimes just keep trying to crank the engine over stops it. Then the truck starts up fine on the next go. This has been happening for about a year, and it has been getting progressively worse to the point where it happens once to twice a week now.

With the starter stuck and the hood open the engine is shaking quite roughly and the belts are all turning very slowly.

Any ideas?

People keep suggesting it's the either the solenoid, starter, or a new one today was the starter's relay. If it's just the starter I'll bite the bullet, buy the part and figure it out but if it's something easier and cheaper I'd love to fix it first!

Thanks for all your help in the past, reading posts have generally helped me fix things before!

Cheers.