O/D OFF FORD RANGER XLT 99

I have a Ranger XLT Aut. 99 The spy o/d off flashes after passing the 55 mph and then makes the change of 1 to 2 abruptly, all the speeds enter and it works well if that speed does not pass, I know that it could be from low transmission oil level to the transmission to the seleenoid of the over Drive, I know it is a common problem but I do not know much about it, someone could guide me a little to know if this is a lot of roll to fix it.