Ed, I am sorry but I still dont get the full picture. the floor joist you speak of, are they the second floor, floor joist, above the sliding glass door header board? The 2z4 is it above or below the sliding glass door and header board? Yes you probably need some temporary load support before you remove the rotted header board...until you can get the new header board in place. Usually any header board above a doorway is supported by what they call Jack or King studs on either side of the header board to transfer the weight of the roof and joist above to the solid foundation below. Sounds a little tricky and complex..I would recommend spending a few bucks to pay an experienced carpenter to replace it for you ...or spend 4 or 5 $'s on a good carpentry basics book that can explain this process to you with illustrations....you might be able to find good help and illustrations on the web as well....Here is a web site that may be a little helpful http://www.lahardware.com/ah4.html#anchor1569452
Maybe other folks on the bulletin board can post some helpful suggestions too....especially Henry in MI or Jay J. Good luck to you.