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Back to the old Grand Wagoneer, it had a viscose coupling in the center so it could be driven in 4-high on dry pavement all day long. I used to just throw it in 4wd all the time, even for rain. Well, I flogged that old Jeep pretty hard, and at about 80 MPH in 4-hi the whole drivetrain would start to make a lot of noise and vibrations. I don't think that this was the best thing I could've done, but it didn't kill it. As for my current truck (97 hardbody, KA24E, 4x4), I've had it up to about 55 in 4-hi and all seemed well, but then I slowed and shifted back to 2wd, cause (as dvdswanson said) if you can go that fast, then you don't need it, do you. I prefer to save it for when I need it. In fact, I often don't even engage 4wd until its clear that I'm not gonna make it in 2wd. You'd be surprized how far you can go in 2wd.
So in my opinion, I don't see any need to go any more than 50 or so, but if in some weird circumstance you needed to go faster, then just use your head and your ears. You can generally hear when a machine is even a little unhappy.
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