'91 Explorer Transmission Problem - Can't start in Drive, Only D2

Hey guys,
Here's a wierd one for you tranny experts. I have a '91 Explorer 4x4 with 300k miles. Not long ago it left me stranded. Wouldn't move in fwd
or rev. Had it towed home, cleaned the filter, flushed, and new fluid. It leaked like a stuck pig. The front seal is bad. So, since I had nothing to loose at this point, I decided to replace the tranny fluid with 20W-50 motor oil in an attempt to stop the leaking. Worked like a champ! I know, there ain't now way the tranny will work with thick motor oil, but it does. Anyway, in the process of playing with the shifter, etc. I was able to get the truck to drive. Here's the trick I learned to be able to drive the vehicle: 1) Put directly into D2 (won't work if I go into D even for a few seconds); 2) At 35 MPH, I can shift into overdrive (D) and all is well; 3) Must drop into D2 if slow below 35 MPH. If I forget to drop into D2 at a light for example, then I have to go back to park, wait at least 5 seconds, then go directly to D2. Reverse seems to be fine. I have the service manuals and have looked over the ECU and tranny info. There is mention of some shift solenoids. I noticed one of these when I dropped the pad, but wasn't sure if it was worth removing and checking or not. Does any of this make sense to one of you tranny guys? If so, what's the fix? The truck's not worth much and I'm not about to pay to have the trans rebuilt since I only drive it about 30 miles a day to work and back.
Thanks in advance for any help.
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I thinbk this thing is a ticking time bomb... one of these times you will be very late for something important.... as you wait for the tow truck...
At one point in time, the repair may have been fairly cheap....
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