I just had this problem a few months ago on my old 88' pickup, more likely than not it is your starter solenoid plunger that is sticking on combined with the contacts that are worn. In my case I would start it and the starter would stay engaged once the motor fired up and would grind horribly. The starter would still go even with the key completely out of the ignition. So my guess is that you need to replace your starter contacts and lube up the solenoid plunger or replace the starter all together about $18(rebuild kit) vs. $90 for a good replacement starter.

Matt