I have a 98 f150 that the 4wheel drive quit working and was wondering if someone had any suggestions as it is snowing badly and my driveway is at about a 45degree angle.

When I try to engage it sometimes I can only hear a relay click repeatedly and no light comes on. And sometimes there is nothing. The fuses are good. I read in another post to check the relays and I do have vacuum on both lines which I know is not right. I was wondering if there is a way to check the relays and if anyone thinks that is the problem. If not any other suggestions would be greatly appreciated. I unplugged the vacuum line on one of the relays and it did sound like it engaged into 4wheel drive but no light came on. If it is the relay where can I purchase it?

Thanks in advance!