You can only rehab a loan if it's defaulted. It looks like OP just has late payments, so unfortunately it wouldn't work in this case. Certainly don't default the loan so you can rehab and get the negs removed 
Rehabbing a loan is a pain and the fees they charge are ridiculous!
Few people on this forum have had luck GWing Sallie Mae, but generally they're pretty strict about their reporting policies. You should still definitely give it a try!
