Bicycle Tutor

I stripped the female part of the cable barrel adjuster on my Ultegra rear derailleur so that the bolt only screws in one turn. Is it possible to rethread it or do I need to replace the entire derailleur? If replacement is necessary, do I need to take the master chain link apart to rethread the chain through the derailleur? Is taking the derailleur cage apart an option if I don't have a chain tool? Thanks

If you just stripped the barrel adjuster, you should be able to just order a matching part and replace it instead of the whole derailleur.

A few even show up in Amazon.

I mispoke, it's not the actual barrel screw, but the unit it screws into.

Something from Jenson's site.
http://www.jensonusa.com/store/imgWrapper.aspx?img=/rd/RD603B00_triple.jpg&type=3
So the part that the black barrel adjuster screws into is what is stripped out? More than likely, it probably won't be able to be re-threaded without losing something. In essence, when something is "stripped out" as you are observing, that would mean the threads are damaged in some way - either the barrel adjuster or the part it's going into. Did you completely remove the barrel adjuster to inspect the threads, both on the barrel adjuster and the part it screws into to see what part is damaged? My guess is you perhaps did that?
"Is it possible to rethread it or do I need to replace the entire derailleur?"
Again much of this involves inspecting it. If the threads aren't in good shape on the derailleur part (i.e. won't be fine if you just clean them up and lube them a bit), then you might be able to rethread it to be a little larger and use a slightly larger barrel adjuster (if you can locate one). Otherwise if it turns out to be a show-stopper and the barrel adjuster still won't thread no matter what you do to thread it in, then it probably needs replaced.
"If replacement is necessary, do I need to take the master chain link apart to rethread the chain through the derailleur?"
From what I can tell of the derailleurs I've seen, they usually aren't meant to be taken apart. The only replacement parts I've ever found for them are the cogs themselves and the mounting hardware. So likely the answer to this is yes.
"Is taking the derailleur cage apart an option if I don't have a chain tool?"
Likely no.
But then again, I could be wrong.

Thanks for your input skyguy. Before buying a new derailleur I decided to look a little closer at the threads. They are stripped enough that I could'nt finger tighten the bolt more than one turn. I brushed the threads as best I could with an old tooth brush and flooded the housing with Triflow. I must have dislodged some small particle because the bolt now threads in fairly smoothly by hand. Now I can spend that money on fueling the engine! Thanks

Then they would have likely just needed chased and not been stripped, which is effectively what you did. Especially with bikes, grime and dirt will get into threads with time and will cause the threads to poorly operate. There are tools to do this job (sized to the bolt), or just running the bolt through will do it if you put enough lubricant (WD-40 or the Triflow you used works) and force on it.
A story to illustrate: I had a nightmare time turning my crank puller the first time I took off my cranks for this very reason. More or less, what needed to happen was a bunch of WD-40 and a whole lot of force. The second time, though, was as easy as easy could get.

If the threads had in fact been stripped, it may be possible to make a repair using a "HeliCoil Insert". Rather than giving a description I'll reference their website where they are described in some detail -- http://www.helicoil.in/helicoil.htm

As an answer to the original query:
" If replacement is necessary, do I need to take the master chain link apart to rethread the chain through the derailleur"
No you don't have to.
You can undo the jockey wheels and leave the chain intact.
BUT:- make sure you note the jockey wheel assembly !!
Sometimes (especially Shimano) the top wheel is different to the lower one and has different bearings in each.
You just remove the small screw from the center of each wheel and they drop out.
You will have to rotate the cage to gain access to one of the screws.
This also releases one side of the cage, but before you do, take a note of the chain run around the wheels first - so that you get it right on re-assembly.