If its as easy as a bent linkage I would disconnect the linkage and shift the t'case into 4-low by hand using the lever on the case. If it still pops out at least you know your pulling the case for a reason. Id hate to pull it and find out after that it was just linkage binding.
Big X2... I had this problem before, and had to clean up, regrease and readjust the linkage. Problem solved once it was working smoothly again and re-adjusted.