mine doesnt gears might be screwed up? are they stock gears? if not, then that could be the problem and i would recomend getting them swaped for a set of 4.10s and a detroit true trac if he can afford it
as for the exhaust leak are you sure thats what it is or could it be a lifter?
99 mustang gt 35th vert - bolt ons
09 f-350 lariat CCSB 5.4 - DD
04 f-350 RCLB 6.0 - sold
---8100 lbs 13.9 @ 96 tune/deleted, stock heads