You seem to know a lot about your father's family. Since Michael Jackson is a common name, an exact date of birth would be more helpful than an approxmate date. You have him about 50, but it would help alot more to know exactly. I don't have any on the list I found that were 50. Where did he live? Where does or did his family live? When did they all live there? Where did your grandmother die? How do you know about her death in 1998? Someone must be keeping in touch with his family if you were born in 1976 and she died in 1998. When did your father leave your family? His two sisters may have married by now and would have different last names.
For Michael Bernard Jackson, these are the results I got:
MelissaData.com | People Summary
I have none with the birthdate that you gave:
MelissaData.com | People Summary
I'm assuming that you know he was born on Valentine's Day, but the part you are unsure of is the year. If so, here is a Michael B Jackson born 2/14/1960:
MelissaData.com | People Summary
There were 4 Nettie B Jackson's who died in 1998. None were born in or near 1932. Not everyone is in the Social Security Death Index, but it does help to have the correct name. This is the site so you can check yourself:
Social Security Death Index Interactive Search
Again, location of death would also help.