Ford Explorer transmission

My 1999 Ford Explorer was experiencing miss-firing of a spark plug and skipping. The spark plug and wires were replaced several times within a 1.5 year period. Just recently the problem became extreme and upon further investigation, the vacuum hose of the fuel line was found to have slipped off and was said to be throwing off the compression of the engine. More resently, the "Check Engine" light came on (again). A diagnostic check showed the same spark plug miss-firing. I replaced the spark plug. Two days later I had the fuel injector cleaned and later that evening the transmission suddenly refused to go into overdrive. The "O/D Off" light came on. The engine continues to run smoothly ever since that vacuum line was adjusted. The transmission has never given any trouble up until the "O/D Off" light came on. It operates perfectly in the first two gears and reverse, but it just won't get the third gear. The transmission fluid was slightly low and dark in color (burned). I topped the fluid off, but the problem persists. Does the vehicle sound as though it will need more than a transmission fluid change and servicing?