99' Ford Explorer transmission/overdrive problems

[Luckycharms65]

I started feeling some slipping, especially at higher speeds, coming home from teaching a G.E.D. class, a couple of weeks ago. It started slipping more often, and finally, I could only drive it only by switching it to in the 2 gear (I have an auto transmission). I have a pan, rather than a valve, so I paid my mechanic to drain most of the fluid, and add more fluid along with some Lucas auto trans, fluid (it completely fixed it last time it started slipping at under 50,000 miles. Now, it only has 63,000 miles and it is improving a bit, but not like last time. I noticed that there was a complete difference in performance if I turned the OD off I did this (after the OD light came on). I have seen so many different explanations as to what is the problem, everything from shuttle valve, solenoid, or an air pressure problem. I must also express, that for over a year, I had my engine light on that was computer diagnosed as a faulty EGR valve. That was replaced, but the light kept coming back on. I was told that I probably have a small air leak somewhere. This is a 99' with only 63,000 miles! This truck should be running for many more miles. I did spend a week driving it short distances to classes with the overdrive off, and in Gear two (that I only used for hauling a few times). What is going on? Please help. I will go to AAMCO as soon as I can. But, I am driving fine for now with the overdrive off.
Any ideas?
Thanks!