Clean the rust off. Try to polish it not scratch it. Maybe start with some light oil and a rag then try BonAmi or a light abrasive. If that doesn't work, a little fine sandpaper will do the trick. If it is pitted it will need to be replaced. Sand in the direction that the shaft turns not the long direction. That way the scratches go in the direction it turns and it will be less likely it will leak.
2006 Superior Blue Trailblazer SS AWD, Stock as a rock
Sold: 2002 GMC Sportside Denali front end with a 2002 LS1, FLT level 5, Yank 2600, Trick Turbo, T76, Nelson intercooler, 60LB injectors, Warbro fuel pump, Eaton locker, Magnaflow, 3.42, Nelson Performance Tuning (speed density).
Sold: 1981 GMC LB RC 1500 2002 4.8L 4L60E 12 bolt 3.73 Richmond Lock Right AC PS Nelson Performance tuning