I worked on a survey crew for three years. We set 18" iron 5/8" iron rebar at each corner, which was easily knocked out. The idea of an axle was a good one, as was the one of setting a 3" iron pipe at the corner. First take 3 spikes or stakes and hammer into the ground around the corner, then measure exactly to the center of the monument so you have a refrence if you replace it with an axle or pipe. A little bit of difference is a whole lot over a quarter mile! Whatever you use, make sure it has a LOT of steel in it. A surveyor uses a metal detector to find the corners, and you want to make it sing if it gets within 5' of it. I would recommend driving a 6' teepost into the ground at each corner until only 3" is sticking out. Even if it is hit by a bulldozer, it will only bend the top foot or so, and a surveyor can dig down and find where it straightens to reset a corner. I would put the teepost every 50' down the property lines. Have a surveyor run the lines again, and set the teepost while he is there so he can shoot it in and note it on a plat. In our area, it cost $100-$200 to have a previously surveyed property redone. In louisiana, there is a 10 year prescription on property lines, which means after 10 years, if a fence line is uncontested, it becomes the new property line. Set the fence on the line, do'nt give away your land!
Jeff