92 Explorer will not go into overdrive

I have a 92 Explorer and it will not shift into overdrive. It is a 4X4 with a 3 speed automatic with overdrive. Although it has had many of the symptoms of the other Explores on this site and all of the fixes have worked, I have not seen this problem on this site. I also have a 91 Explorer that I bought for parts and would like to get mine shifting correctly before I haul the other to the scrap pile. Any assistance would be great!