I've been able to shift both M/T and A/T 2.6 Gen 1's between 4HI and 2HI, once the hubs were locked, at pretty much any speed (on appropriate surfaces, of course).

You can drive in 2HI with the hubs still locked in case you feel you might need to go into 4HI. They'll stay locked unless you back up, like FastEddy said.

I'd really use caution going into low range, though. Full stop, in neutral, may have to let the truck roll just a bit.

Of course, don't try it on your truck if you're not comfortable with my technique. I just know it worked for me.

John B.