starter will not disengage

I have a 1993 Explorer, I have replaced the starter twice and replaced the starter Relay but I still have the same issue, my starter will not disengage once the truck is started. It has no issues when starting, it turns over quick and starts strong. I can sometimes turn the ignition when the truck is on to the start position and it seems as if the starter disengages but as soon as I return the ignition to the ON position, the starter reengages... What should I check next?

mr23mr sounds like your ignition switche is shorting out internally! Disconnect the ign wire from the relay and check it for power with the key in the run position. If its hot in the run position you've got a short in the switche.