Don't let these other guys scare you. Yeah, you might screw it up, but hey, if you have the time and patience-go for it! As long as you are ok with the idea that if you mess it up you may have to pull it out and tear it apart again, then you can only learn from trying it yourself.
One thing you will find is the 4R70W is very modular (ie, pump assembly, valvebody, clutch packs), and if you are doing a simple teardown to just swap the output shaft then you aren't dealing with a thousand individual parts, but groups of parts that come out self contained. From there, the level of difficulty increases based on what you want to do (simple rebuild-clutch plates and shift kit, performance mods-adding clutch plates, upgrading internal parts; each process makes it more difficult).
If you are going to try it, there are a couple things that are VERY helpful to make sure you do it right.
1) Contact ATSG as mentioned and get their manuals/videos.
2) Contact Helm Publishing. They are the Ford's authorized publisher. When I rebuilt mine, I got a teardown/assembly binder for the year of my tranny-extremely helpful!
3) Spend at least a week or two on as many boards as you can find that are oriented towards working on these trannys. Find out all the tips and tricks you can from the experienced builders. Also use this time to figure out if you just want to swap the shaft, do a mild rebuild, performance build, etc., and then plan ahead and get all your parts ready. Also find out what tools you will need to buy or make.
4) Once you tear into the tranny, try not to walk away for a month and then come back to it. The more fresh the process is in your mind, the easier assembly will be, and mistakes will be minimized.
5) A good size, cleared off bench is a must, but I found it was most helpful to tear it apart on the floor and work on individual things on the bench. Go get a cheap tarp, and lay out the tranny as you take it apart like an exploded diagram. This will make life a lot easier.
6) As you take it apart go slowly-springs and clips love to pop out, and then when you're putting it back together you'll find asking yourself, 'Which detent did this clip anchor to?' Pay attention to the orientation of things as this is crucial as you re-assemble it. Pay attention to the orientation of EVERYTHING-bands, clips, springs, clutch packs, etc. Even take pics as you go along for reference.
BTW-shameless plug. When I was doing mine, I had a hard time finding stuff online as it related to rebuilding AOD/4R70W's. So, I started a Yahoo group specifically about that. We now have about 240 members and several of them are transmission rebuild shops. It's really become a good repository for knowledge about these trannies.
http://autos.groups.yahoo.com/group/AODominators/
Good luck!