Not in low traction situations where one front wheel has none.
4WD trucks aren't really driving all 4 wheels because the differential allows them to have more manageable highway manners so the wheels on the same axle can turn at different speeds while negotiating a corner.
When you add a rear locker and turn it on in the case of the elocker, both rear tires are driven at the same rate, regardless of traction or turning. It's effectively like a spool when engaged.
Which means....you have two tires driving in the rear, but up front, you may only have one if say the passenger side tire is in the air or is spinning helplessly in mud. In that case, you really have only 3 wheel drive, but the truth is, the one in the air that's spinning isn't doing you any good in terms of forward motion. What you then have to hope happens is the rear "pushes" the front forward enough for the front end to regain traction. The better solution is a front locker....then all 4 wheels will pull (with say an ARB for example and to simplify the discussion).