Looking at the valve body flow diagrams, there doesn't seem to be any way for the OD to feed the reverse band or intermediate band to apply it.

Those are the only two bands in the front half of the trans that could come on to accidentally bind up the front half.

The 2nd gear servo always has pressure to the apply side. The only way that it can turn on, is when the pressure from the disengage side of the servo is removed. I can't see how the 3/4 shift valve solenoid could cause a removal of that pressure.

The only way the reverse band can come on is through the manual valve for reverse or manual low. If it were being back fed from the rear drum, it would be locking up in 3rd gear also, not just OD. Again, I can't see where the OD circuit could even get into the reverse band or intermediate.