Thank you for so many responses to my question about Sadie's death certificate. You all asked many good questions about what I know. Here is a summary of information I've gathered:
Obituary-"Sadie J. Dunham...died Sunday evening at Flow Memorial Hospital..." Denton Record-Chronicle, Tuesday, 29 April 1980 (our family was in and out of the hospital at the time near her death; not sure who was there closest to her death since I lived out of town)
Funeral Card-Date of Death: April 27, 1980 (I was there at the funeral.)
SSDI: Last residence: 76201, Denton, Denton, TX; died April 1980
Schmitz, Floyd, Hamlett Funeral Home, Denton, TX (this found in records housed at Denton Funeral Home): Sadie Johnson Dunham-died 4-27-80 at Flow at 9:20 p.m. (Other information about her listed here for the obituary)
So, as you can see, I have good evidence of her place and date of death. Her daughters, who handled funeral arrangements, are now deceased, but this wasn't that long ago. My aunt, sister, and I were also there through some of this.
I do also think that my great-grandmother took care of the dispersement of all her worldly goods BEFORE she died. She was a forward thinking woman! I found a deed where she sold her house to her daughters less than a week before she died. So there might not have been a need for a will probation. There wasn't one listed in the Denton County records.
Okay, all that said, I just received an e-mail from someone in the Denton County Clerk's office who said "While rare, it is a possibility that the death certificate was never filed with us. There is an option to file a delayed death if that is the case and it is not on file with Austin..." He went on to give me information about how to verify with the state that there is no record on file. So I'll do that next.
So much thanks to all...I've learned some new things...and I'm not sure how Sadie was listed on SSDI without the death certificate, but maybe the state will search their records for me to see if one was actually filed. I truly wish my grandmother was alive to tell me what in the world went on here! :)
If anyone is interested, I'll post later as to what happens. Genealogy is certainly not a dull hobby!