I would check the rear diff. U replaced an axle but from what i read the spider gears werent replaced and u dont know the extent of damage it did to the gears. Also....might be as silly as a dust shield on the rear brakes....
1996 Mustang GT, 17x9 front and 17x10.5 cobra r replica rims, 255/40/17 front 285/40/17 rear at the moment,
o/r x-pipe flow master 40 series dump downs, cai/ram intake, 75mm c&l throttle body and plenum, pi heads and intake swap, msd wires, stock gears, untuned at the moment, no dyno numbers for now.