4x4 won't engage

I have a 2003 s-10 I have replaced the vac. switch on transfer case and the vac. actuator, I have checked it and it is pulling the cable but still no 4 wheel drive? the 4 wheel drive buttons on the dash will a steady 4 Hi. light, when I push 4 Lo. the 4 Hi. stays lit and the 4 Lo. blinks, what should I check next????