98 Ford Explorer Blinking O/D (overdrive light)?

If the overdrive cancel indicator light is blinking, it means the PCM has detected a problem within the controls or circuits that operates the shift solenoids, and/or one or more if the "inputs" from the transmission has fallen "outside" of the normal operating parameters of what is considered normal. The only way to fix this problem is to take it in to a Ford dealer, or a transmission specialist with a scan tool that can communicate with your vehicle to determine what fault, or condition is present. Once the code(s) are pulled, you'll need to have appropriate pinpoint tests perfromed to determine the root cause of your problem. Hope this helps.