Interesting point. I've been disregarding automatic transmissions for so long, that I wasn't even aware the 4L80 was available that early.
Yep, in the Squarebodies, 91 in the only year. Which is a good thing cuz if you found a 91 in the junkyard set up for 4L80-E, then there you go. You now have parts to retrofit and earlier model. Crossmember, wiring harness, shift indicator and such. So all 91 Crew Cabs with Auto Trans, both single and dual rear wheel, even the 3/4 ton Crew Cab since it's a 1 ton frame too, will be 4L80-E, and then the 3/4 Burb in either 2 or 4 wheel drive. What's very elusive is the 91 1 ton 4x4 with the 4L80-E and the NP205 transfer case. But yep, for 91 only.