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I have and AOD in my bronco now and want to upgrade to a 4R70W. I need to know if I buy a 4R70W out of a 4WD explorer will I still have to replace the tailshaft to mate it 2 my Dana 20.
Yes. The 4R70W will have a 6 bolt circular adapter and a 31 spline shaft. It won't work that way with a D20, but an NP205, Atlas, or STAK will bolt up. The Atlas & STAK will most likely require a 1" spacer, because the 4R70W shaft will be too long.
The AA AOD shaft is 27 spline.
You will need to transfer that over to the 4R70W.
The 4WD version of the 4R70W is a 31 spline (full size truck) output, so it won't work.
Being that you already have the AOD, the 4R70W swap should be easy. Transfer case moves back a little less then an inch, so if the driveshafts have enough travel left in them they could go in as-is. Flexplate and starter will carry over as well.