I can't pull the picture up but from the description, if by traction boards, you mean the offroad aluminum ones to get you unstuck...your original plan should be fine.
If it's only 50yds, you could also look at picking up a 200' anchor line (1/2 or 5/8" should be enough). Secure it well to the 2wd on dry ground (you don't want it coming off under tension). Then have the 2wd take up the slack and apply some force before the 4wd starts. The anchor line will be cheaper and easier than several sheets of plywood. (don't try jerking it out)