OK, after almost 40 years of driving trucks with 205s and locking hubs, let me say this.
Under normal conditions, I can drop my truck into 4wd at 70 if I want to. No synchros involved.
If you are running different size tires, different diff ratios, spinning the rear tires, or turning a hard circle, then the gears are going to grind.
In a straight line run, with all things normal, the difference in RPM between the front and rear shafts is tiny.
All that is with the hubs locked. If you have auto hubs, or the hubs are unlocked, then the synchros will spin up the front shaft so you can shift on the fly.
If the hubs are locked, and you are spinning the rear tires with the front not turning, then no synchro in the world will match the spins.
To lessen the strain on my drivetrain, I often run with the hubs locked but the transfer case in 2wd.
When I need more pull, I pull the lever back and go into 4wd. If my rear tires are spinning, I back off the gas just long enough for the rears to match speed with the fronts, pull it into 4 and get back on the gas.
There will always be a slight difference in the speed of the two shafts. When you are driving on hard dry surfaces, this will cause the drivetrain to bind up until something either slips or breaks.
If you have overbuilt your drivetrain like most folks here do, then usually the tires slip.
Its still a big load on the system, I and just don't like it, so I shift in and out a lot.
On my old 150, I once pulled out on pavement not realizing that I was still in 4wd. It bogged down and stopped about 200 yards on.
I could have added more power and it would have kept going by slipping the tires, but I backed up and shifted out.
