I asked this question a long time ago and did some research on it. I only removed one of the existing ones so I was able to use plywood with screws spaced about 2" apart to achieve the "racking" strenth.
You can also take a piece of 1x4 and cut the ends at a diagonal. Trace it out along the studs and chisel the wood out.
If you went with the 1x4 you'd likely have to consult an architect to determine the proper placement.