The output spline count changed sometime around 1989-1990 - if you want to be sure of getting a 23-spline (stronger shaft anyhow,) get a 1991-up. There weren't any mechanical changes I can think of in the production life of the AW4.
You'll usually have to pull apart the transmission and transfer case to verify the spline count.
Also, bear in mind that the AW4 is a Toyota box - giving you a few more options for sourcing a donor (Pickup, 4Runner, RWD Van, and maybe Land Cruiser.) I don't know what transfer case would be used with the Toyota OEM box, but you could always swap the transfer case adapter off of your slushbox and reuse your transfer case (assuming the splines match.) If the Toyota box is a 23-spline, grab either a later transfer case, or get the input set from a 1991-1995 or so transfer case (they changed the cut of the input gears sometime around 1996, and they won't mesh properly.)
Oh - and I think there actually is an AW5 - as a five-speed auto used in RWD Lexus SUV and maybe Toyota SUV as well (I won't swear to that one, tho. There's a vague tickle, but that's all I'm getting.)
It's possible to overhaul an AW4 successfully - I've done it once myself - but you'd better have some AT overhauls under your belt already. It's a fairly picky box to assemble, but it's simpler than most others I've worked on in the past. I put 50Kmiles on the transmission before I lost the pinion bearing in the rear axle...
