Whenever you get the first sign of a problem, you take it to the shop. Do not wait until you get a blinking OD light.
I purchased an Explorer used, cheap. The OD light was blinking, but I knew the tranny was going bad. The previous owners just drove it with a bad tranny. I got a full rebuild including, new EPC Solenoid, rebuilt valve bady, clutches, and torque converter for less than $1,800.00. The torque converter came apart throwing crap into the valvebody.
Find a tranny shop that is known, don't go running for the dealer. You will end up paying more than you need to pay for the same repair somewhere else.
Tom
p.s. The quote from the dealer, employee cost was over $2,200.00. By going to an independant shop, I saved a minimum of $400.00. And the dealers quote never included a torque connverter.