No, It shouldn't be able to pop out even under load. the spine should normally be 2" long and usually has a C clip which is what pop's back into place.. The spines are in a axial direction, it shouldn't be affected by rotational force unless it was'nt properly installed at a prior time. To remove half shafts you usually have to use a pry bar and hammer to get the C-clip to let go inside the diff. Pry bars have a lot of leverage! Was probably not fully installed into the diff housing. Was there any gear oil leak? might want to check your front diff fluid level to be sure! Good thing ya found it! <img src="/forums/images/graemlins/wink.gif" alt="" />