Matt,
I have an '89 Runner, 3.0, auto, 4.10 gears, stock tires, Downey Exhaust system, rebuilt motor last spring (dropped valve seat).
To answer some of your questions:
Mine has 130k miles on it, works fine, that is the only data point I have on durability. The A340 trans has been put into many Toyota and Jeep/Chrysler rigs. Mostly without the hydraulic actuated/integrated transfer case that the Toyota rigs come with. (link to history below)
The torque converter locks up at freeway speeds. Good gas mileage? I get 20, maybe 21 on the freeway at 65 mph. In town it drops way off to 15/16 mpg.
Gas mileage is worse, from what I hear, as compared to a manual trans.
I have pulled a boat with mine several times. You pretty much keep the O/D off the whole time and have to shift into second often. Not a towing machine by any stretch. The 3.0 doesn't hit it's power band until 3k or so. You end up using second a lot to keep it in the sweet spot.
As for shifting action, it hunts around to much for my taste on hills. I think this is mostly due to the low output of the 3.0. It will go in/out of O/D frequently, lock unlock the converter, lots of RPM jumps, drops, jump, drop. Gets a little annoying.
There is that cool power button that you get. It changes the shift points and when the converter is locked up. When towing I usually have the power button on. Power On!!! Well, not really as impressive as it sounds.
Now, if you ever are thinking of putting a doubler transfer case into your rig, buy the manual transmission. There are a couple of adapters for the A340H to put in a doubler setup, but it really costs a ton more money. The transfer case on the A340H is kinda-sorta integrated into the transmission. It is not some separate bolt on unit like you normally see. They share some fluid flow and removing it can really screw up the trans. Gearslug makes an adapter. Also, Inchworm has one.
In the end, wish I had bought the manual version. But, I am 6'4" and have a hard time with a clutch in these little tonka truck rigs. If I had the stick, my left knee might be telling me I should have bought the auto. As it is I cannot see stop lights when I sit up straight, just the top of the windshield.
Links for you on the A340H:
Short history of the A340 series Good details on the trans.
