Assuming all of the components are in good shape and you do not have any u-joint angle or inbalance issues, you can drive as fast as the road conditions dictate. As mentioned, if you really need 4wd than the roads will be at least somewhat slick and therefore the speeds will be lower.
I've done a lot of long-term durability and evaluation testing for major auto manufacturers, and a lot of this testing includes driving part-time 4wd trucks locked in 4-high and 4-low for long periods of times while on dry asphalt and concrete surfaces at speeds approaching 100 mph.......I'm talking thousands or even tens-of-thousands of miles on lot's of different vehicles........and I rarely ever see a failure that can be contributed to running in 4wd on these surfaces.
On my personal daily driver / towrig I've ran for over 100 miles continuous in 4-hi while pulling a 9,000 lb. trailer when the roads conditions were constantly changing......some areas were snow covered, some were icy, and some areas were clean dry asphalt.
In general, 4wd does NOT help braking during the typical braking manuever on the road. Going down a hill in 4-low on the trail, sure it helps, but not on the road. The wheel brakes, again assuming in good shape and appropriate size, should have more than enough power to slow the wheels down...and be much quicker and more controllable than any normal compression braking.
however in 2 or 4 wheel drive.
I drove to work this past Sunday in 4HI at around 45-60 in the ice and sludge. A friend of mine told me I would tear up the t-case doing that. I figured they called it 4HI for a reason,so use it. Hell if it breaks it needed replacing anyway right.
