I didn't know this either until last weekend.
1991-1993 4L80E units have a wiring harness unique to them and not compatible with anything newer or any of the aftermarket controller units. Besides changing the internal wiring harness, these years have to have the solenoids inside changed over. Figure $200-$300 while doing a rebuild to get that done.
1994 has the modern solenoids and wiring harness already installed. It is also the last year of the mechanical rod shift linkage.
1995+ has the cable shift linkage and bosses on the side of the transmission to hold the GM reverse light/neutral safety switch housing. They moved it from the column in 1995 when they added airbags.
Basically, anyone with a 1993 or older truck looking for a replacement needs to find a 1993 or older to allow it to plug in and work. Anyone with a 1995 and newer truck needs a 1995 and newer to hook up the shifters and wiring. That leaves 1994 as a unicorn model. It will only bolt without modifications into a stock 1994 or be used for clean slate swaps like I am doing. I know the shifter switching from linkage to cable can be gotten around. However, there are no mounts in the side of the 1994 to take the neutral safety switch and from what I found, the shaft itself is different and needs to be replaced if used on a 1995 or newer.
I have a 2wd 1992 4L80E that came with all the parts from Warthog. I have traded in the last few months for a proper aluminum 4x4 4L80E transfer case mount and output shaft. Then I started calling transmission shops. Since I was needing a new internal wiring harness and solenoids replaced, plus a rebuild. Several told me they couldn't convert to 4x4 and they got hung up on as being idiots. The few remaining said $2,000 plus anything they find wrong inside. I did find a retired guy referred to me by several people who said it shouldn't be more than $1,200 and about a week to do the work.
That is why I thought it was a scam when I found a craigslist add for a 4x4 1994 at only $400 last weekend. I texted the guy and he kept writing back that it was a good unit just pulled from a 6.5 TD truck with a cracked block. He also kept asking what year truck I had. 1987 had him confused. He finally gave me his address which was 135 miles away. The Blue Bomber and I took a road trip last Monday to find a guy with a single bay metal building in his back yard. His wife has a regular job, he takes a single vehicle apart putting everything online for sale and watches the kids. He scraps what is left to start on another one. He knew exactly what he had and why it wouldn't bring normal prices. I did get to look at the cracked #8 cylinder wall, dump transmission fluid which was the proper color with the proper feel and smell from the cooler, lines and converter. Being a diesel converter which came with the 4L80E was nice as well.
He didn't have the flywheel cover though. He said it wasn't on the truck when he got it. Anyone have one of the aluminum 4L80E flywheel covers gathering dust they want to get rid of cheap?
I really thought about even mentioning the 1994 4L80E difference last night. Yes, I have what I need for this project but I still have a few M1009's that might want one someday and I don't want the prices to go high on them too. However, with several hundred thousand made in 1994, I doubt this internet site will corner the market on them enough to drive the price up. Enjoy the knowledge.