What does the rear wheel speed sensor read If rear axles do not come with tone ring?

I have the vehicle listed above n51 e90 sedan. I replaced wheel speed sensor 2 times now dealer shop telling me my axel is corroded. So, I ordered 2 rear left driver side axles. One from carpart.com and from advance auto parts. I hope one fits my car, please help. I noticed they don't have ABS rings like the front axles. What does the ABS sensor pick up then on the axle? Is it really my axel that need replacing or my bearing or maybe both? ABS lights are off till I give it gas. Brake pedal will pulsate until the ABS lights turn on after giving it gas. Then the brakes work fine but ABS and traction control lights stay on. I get a p1774 code meaning no signal in rear left wheel speed sensor. Please help axel or bearing maybe both? ABS reluctor ring/ABS ring/ tone ring missing on working and non-working rear axles? I also can't find any parts I can order online coming with one. So, what does the ABS sensor rear please help.