I don't think you can run 60 at a moderate RPM with a 2 speed. All two speed rears I know of are slow and slower as the two gear choices.
I suppose you could have an overdrive transmission or two, or some really realy tall tires but lack of power will probably keep you under 60 most of the time, and all of the time with a good load. Mechanicly the rear would be fine at higher speeds, if you could get it there. And with brakes in good condition should even be somewhat safe if you use common sense.
on the power thing, I'll give you an example. My 1 ton has 4.56 gears (Faster than anything in a two speed rear) and I can haul 4,000 pounds just fine in the back if it's not to hot outside, and I may get up to 50 if I am lucky. That's with a 235. But for some reason if I pull a 18' flat traler empty I can barley get out of third gear on level ground.
Again, There is no substute for cubic inches, Except a supercharger of course,.... think Detroit Diesel here...
Any way you look at it, a 235 or a 292 and a loaded 2 ton truck are not going to go fast..
The link below shows my axle swap stuff, and some of my soon to be diesel swap.
