4x4 low range

i own a 97 z-71 with a 5.7 and 4L60-E. when shifted into four wheel drive hi it works fine, but in 4-low, it doesn't want to shift properly. the motor revs high, but will not shift. if i shift manually from first to second, it hits very hard into second. it will not shift into third or fourth in low range. the transfer case switch has been changed and it is a manually shifted transfer case not the electronic version. any advice would be appreciated. thanks.