As im backing up and continuously tapping the brakes i can feel my front axle move back and forth. Any ideas?

 

Check the control arm bushings for wear and damage.

 

yup, prolly yer control arm bushings.

but also check your brake caliper bolts.

 

I would have vote for control arms, either the bushings or the mounting bolts. Get a creeper, roll under your rig and check for anything that's loose. It may be a few things and not just one, don't go into it with tunnel vision thinking control arm.

 

either have someone look while you have it in reverse and rocking the brake pedal, or get someone to sit in the drivers seat and rock the brake while you inspect the control arm bushings. start by checking the frame mount ends.
that's where mine were shot and with a friend, helped me diagnose mine.

but have also come across the brake caliper knocking a few times also.