Replacing the injector seals will have zero effect on an injector that might be leaking, unless you've got obvious signs of fuel leakage from the fuel rail and the accompanying fuel smell in the engine compartment. I'd be more inclined to suspect a dirty injector or a bad fuel pressure regulator. Like John B. said, sending them out to be cleaned and flow tested from a respected company isn't a bad idea if the vehicle has got some miles on it. Bad fuel injectors are uncommon on Monteros, but if the vehicle sat for long periods of time there could be varnish gumming up the injector internals. The good injector places will give you flow and spray pattern data from both before and after the injectors are cleaned so you'll know if there was any leakage and how badly the injectors were clogged.