The weak points of an AOD seem to come from the high performance/street mustang crowd. I've talked to ALOT of people that run AOD's in Bronco's and BAX is the only one I've ever heard of that's had any problems. Most seem to think it's a pretty decent trans.
As far as sourcing a low gearset, scavange a cheap 4R70W for the gears and upgrade parts.
After searching for a builder for the AOD I finally found an old school biker type, that was highly recommended by several people. He told me the AOD is nothing special as long as you keep the tolerances within spec (he means use the book) and use the seal install tools (or be careful with a pick and dentist tool), other than that standard tools and feeler gauges. The seal tools are usually on ebay for $40-50. A one piece output shaft might get you strength for full throttle 6,500 rpm, 500 horsepower, 400ft lbs of torque shifts, but who really does that in a Bronco(besides BAX)? And you loose the 'locked' torque converter in overdrive, which means more heat on the highway... Plus then you have to buy a new torque converter.
If you think your confident enough to think you can tackle an AOD you probably can. I was all about doing my own, then found a fresh one with about 1000 miles on it, so I don't have to rebuild mine until I do the low gearset swap myself...