There's something fishy with this rear axle

[bcfishon]

I've got a rig that Im working on and do not know any history on it and during the test drive the right rear brake will lock up prematurely and there is something grabby going on,its a pulsing rhythmic grab. Also noted a severe shake in the back area of the rig like a wheel is bent or terribly out of balance. Up on jacks and upon inspection it appears someone has converted the rear axle to five from 6 lug and the center hole on the five lug brake drum is too small for the end of the axle flange to go through. They had washers on the right side to compensate for this and nothing on the left. I'm wondering if someone drilled the axles and used what brake drums they had available. Not sure where to go from here so wanted to hear what others might suggest before I dive in,thanks