4WD low has a high gear ratio, so can be hard to change on a hill, remember the transfer case is connected to the wheels, clutch does nothing to take pressure off that, thats between engine and trans only.
There is an electric transfer case clutch for "shift on the fly" but it doesn't always release pressure from wheels.
And brakes just lock it in gear because wheels are locked.
I.E. park on a hill in 1st and turn off engine let 1st gear hold your position, then try to pull it out of 1st, foot on the brake NO CLUTCH, can be very hard to do, even on a slight hill.
This is why level ground makes it easier to switch out of 4WD low
So not to unusual
Shifting into 4WD low may also require a roll forward or back when on a hill for the same reason, the gears need to mesh and if not aligned then they need to move slightly one way or the other, on a level surface there is enough play in the drive line to do this, but on a hill the alignment may need to go in the direction of the hill and can't because wheels are locked and all the play is gone.
4WD high is a much higher ratio so you hardly ever run into this.
Could be shift motor is starting to wear out or slightly out of adjustment, yes there is an adjustment on the motor to align motor sensors with transfer case shift positions.
This thread shows how the shift motor works when you chose 2WD, 4high and 4low: How to: Rebuild 4X4 Shift Motor | Ford Explorer and Ranger Forums - Serious Explorations
You see the "sensor wheel" in the motor, that moves with the motor and transfer case shift shaft, when wheel gets to a certain position 4WD module says "that's in 4Low", it doesn't know if transfer case IS in 4Low it just knows sensor is at the right position for 4low, which is the adjustment.
Light will flash if it can't move sensor into that position, so try rolling forward or back if that happens, so motor can turn all the way into selected position