MF most of the time, I find autofocus is rubbish on the D70, probably because it doesnt have many focus points ? I cant get any decent depth of field on the 18-70mm glass, it just takes a picture as you see it with the naked eye, ie focuses on everything. Use MF with the 50 1.8 and get better results than AF.
Wierdly i borrowed a D40 once and i got focusing and DOF just like a 50 1.8 with the 18-50mm kit lens, on AF. Cant do it at all on the d70 and AF.
Sounds like your D70 or 18-70 is broken.
I had that combo and AF worked perfectly and was quite snappy.