I'm running 33x9.5s with stock gears (4.56 most likely, or possibly 4.88, I haven't checked.) and wheels. There's no need to change gears if you're staying on the road most of the time.

Watch the clearance, though. If you don't get more width on the front axles I'm guessing you'll probably have some _serious_ rubbing going on, especially when you go around a corner. With the 9.5's I am having a little bit of rubbing with full articulation on corners. But that's rare, right? <img src="/forums/images/graemlins/coolg.gif" alt="" /> Just gotta watch it.
Someone else could comment on whether it's different on your '85 p/u than my '93 4runner for clearance width.

I haven't tried airing down to see how well the tires stay on the wheels, nor have I done a *lot* of trail driving since I put these on, mostly hill climbs, but I had no problems. I would think the 9.5's stay on the wheels better at low pressures than 12.5's.