I have just found out that my ssdi was approved. I haven't even received my award letter yet. I have a defaulted student loan and I'm aware they can garnish my monthly check (once I start receiving it.) I'm going to apply to have my student loan discharged for total and permanent disability but it will take time, if I am approved at all.

My question is; can they garnish the lump sum of back pay that I will receive from ssdi (because of waiting 2 years to get approved for disability.)

You need to run this past your disability attorney if you have one handling your case. Since this is a government student loan you defaulted on the possibility is very high this could happen.

It is possible, or they may do a percent of it,

Student loans can not normally be discharged even with bankruptcy. And student loans garnishment is normal even for disability.

Sorry, but yes, they can take your entire back payment check for not only defaulted student loans, but any type government loan, these type loans are NOT unforgivable and can't be written off, unlike a mortgage or bank loan can. If you owe back child support payments, they can also take your full back payment, even if it doesn't cover the full amount. Then the gov't will garnish your monthly income. Didn't happen to me, but this all happened to my brother in law, they not only took his back payment, garnished his monthly SSD, but froze every bank accout he had.

So be warned, it can and probably will happen. Just glad I did not have any of this when I got my SSDI, otherwise I'd be in the crapper like my brother in law.

Now if you have a relative you can trust and get the back payment before the gov't finds out, you could always stick it in their account, but you better have some tight bonds there, because your name can not be on the account anywhere, otherwise, they'll do the same to that persons account.