transfer case kicks out of low

[tbro47]

I have a 74 bronco with a 302 wiht a c4, dana 20 transfer case that is twin sticked with front and rear detroit lockers, When in 4 low the transfer case kicks out of low gear in the rear into nuetral which really sucks when your on a hill. If anyone has an ideas as to the problem i'd much appreciate any input I can get.