1993 jimmy

my jimmy wont go into 4 wheel high i can get it into 4 low the light will come on then go off and it stays in 2 wheel drive its a new car to me and i used 4 wheel drive a couple of times without any problems it just started doing this for no reason?

When this happen to my 93 S-10 Blazer it was the vacuum switch on the top of my transfere case...GM only part as far as I know