westy44runner

I own a 94 4runner, auto v6. It has almost 150,000 miles on the truck. Its all stock, except for slight lift but I have never had any transmission problems at all until this...
I was traveling up towards Payson, AZ to Reno Pass trail, engine was in high rpms most the way up the mountain grades, then we got to the dirt road and headed up it about 10 miles, slow going. had the truck in 4HI most the way since it was slick/rain/snow. All of a sudden my AT oil temp light came on. I drove another 1-2 miles then pulled over (should have stopped right away). Stopped the truck and checked fluids and all and everything looked ok, started it back up (should have left it idling i know) and the light went off - we traveled further up the mountain to the trailhead where I lost all HI gearing. 4HI, 2HI were not working at all! 4LO worked just fine however. Towed it back to town, the next morning I had the tranny flushed and new fluids put in. works great since then around town, have yet to go back to the trail.
My question to those of you knowledgable about these issues is what type of reliability it will have in the future? Is my transmission shot the next time I try to wheel it? I realize that when traveling 30-60 mph the tranny keeps cool and is more efficient at cooling itself but when 4wheeling its alot rougher on it. Any insight would be great...thanks guys...

westy44runner

...also I do plan on take it out wheeling on of these next few weekends with another vehicle (tow vehicle) just in case...we'll see what happens...hopefully its not fried..