200K miles on a 4L60 is really pushing it. More than likely you have a pump problem or possibly the forward clutch pack is gone. In reverse you have a pressure boost circuit from the pump which is probably why you get reverse but not forward. But the forward control unit could also cause the problem. (really, there are several things that could cause this problem)
Either way, the tranny will probably need rebuilding. Like I said above even if you are able to talk the shop into fixing JUST what's wrong with it they won't gaurantee the job because the next control unit (with 200K) will go out, and so on.
Replacing the torque converter is not an option. The converter is full of little dirt catching vanes that can't be cleaned without opening the converter up. All that contamination will flow right back through the valve body and pump and cause premature failure. The torque converter MUST be replaced. (again, most shops won't gaurantee their work without a new or rebuilt TC)
I'm not a big fan of putting in a bench job rebuilt tranny. One, it hasn't been road tested, two, at least you know what your tranny has and has not done for you over the years (history) and three, you won't have to worry about a porous case or sticking valves, etc (see two). But I know folks who have chosen to stick in a bench job and the transmission, most of the time, holds up just fine. Then there are those who get ........(be nice).