Jeff is correct!
1991 was the first yr for the 4L80 and it was only available in 2500 with 2wd & 4wd. TH400 was no longer offered.
700R4 was for 1/2t trucks.
Odd side note, the 4spd manual was no longer an option in Suburban for '91.
The 1 ton V3500 pickup still got TH400 though.
Have a read...tells you more than you ever wanted to know about '91 Subs...
https://www.gmheritagecenter.com/doc...t-Suburban.pdf
And is link for '90. You can compare what GM dropped off the option sheet.
https://www.gmheritagecenter.com/doc...t-Suburban.pdf