if your running a spool -it- can cause a dragging of the tires when making turns in the pits and track---you could have a bent housing or one that was not jigged right when narrowed and rewelded.
how long did it take to wear the inside surface[did it also wear off the very edge on the inside of the slicks]
jack the car up onto stands that are the same distance off the floor exactly/
them carefully measure across the tire surface.
pull a string line across the center of the housing on the backside and check in different spots to make sure it is straight
check ALL welds on the housing for ANY cracks or stressed spots [sometimes they show up as rust lines along the welds]
good luck--[i put a rear brace and a bottom brace on all 9 inch rearends that are going to be abused by horsepower/wheelstands/ or high rpm starts] 8)