Have 77 f250 highboy 4speed when under load the transfer case tilts forward enough for it to change from 4h to 2h. I have seen one with bar from bottom transfer case to frame , have done that with rubber at frame mount, still will pop out when pulling steep boat ramp with big boat . Help

 

Out of curiosity, are all 4 tires the same make, model and size? If not, that could be your problem. I have over the years had people complain about their trucks no staying in 4WD only to find that they have different tires on the front and rear. Even the same size tires from different MFGs are actually different in size. This difference can cause the vehicle to "WALK" out of 4WD into 2WD due to the driveline binding.