8&3/4 axle fitment problem

pumpkin out. OE SBP 8&3/4 axle will slide in 3&1/16" and the dr diff BBP axle will slide in 3&3/8". looked in both sides & still see the round "flat" wall. sticking a tape in, the depth of round flat on the one side I checked is 3&3/8". nothing looks out of place. RR