I hate to say this but your converter may be the problem, was there any noticeable debris or foreign broken parts in the pan this time?
If so maybe it is time to have the torque converter looked at
The last simple thing I can think of that could cause this problem is the shift linkage, I set mine by starting in neutral and adjust the cable end so it drops into the hole in the shift arm easily, no resistance, once I have it adjusted for neutral I then check it in all the other forwards gears, some shifter cables will not allow you enough slack in the cable in reverse to have the end slide in and out of the arm easily
Let us know what you find out please