My wife's 5th Gen 2019 RAM 1500 4x4 Bighorn, in the owners manual, says NOT TO EXCEED 25mph in High and NOT TO EXCEED 10mph in LOW. If you do, the computer kicks in, and brings you back down. Dealership confirmed that is EXACTLY what it is designed to do.
Not sure what owners manual you are using, excerpt from MY 19 manual:
When operating your vehicle in
4WD LOW, the engine
speed is approximately three times that of the 2WD or
4WD HIGH positions at a given road speed. Take care not
to overspeed the engine and do not exceed 25 mph
(40 km/h).
This electronically shifted transfer case provides five mode
positions:
• Two-Wheel Drive High Range (2WD)
• Four-Wheel Drive Automatic High Range (4WD AUTO)
• Four-Wheel Drive High Range (4WD HIGH)
• Four-Wheel Drive Low Range (4WD LOW)
• Neutral (NEUTRAL)
For additional information on the appropriate use of each
transfer case mode position, see the information below:
2WD
Rear-Wheel Drive High Range — This range is for normal
street and highway driving on dry hard surfaced roads.
4WD AUTO
Four-Wheel Drive Auto High Range — This range sends
power to the front wheels automatically when the vehicle
senses a loss of traction. This range is used during varying
road conditions.
4WD HIGH
Four-Wheel Drive High Range — This range provides
torque to the front driveshaft (engages four-wheel drive)
which allows front and rear wheels to spin at the same
speed. This provides additional traction for loose, slippery
road surfaces only.
4WD LOW
Four-Wheel Drive Low Range — This range provides low
speed four-wheel drive. It maximizes torque (increased
torque over 4WD HIGH) to the front driveshaft; allowing
front and rear wheels to rotate at the same speed. This
range provides additional traction and maximum pulling
power for loose, slippery road surfaces only.
Do not exceed
25 mph (40 km/h) in this range.
Five-Position/On-Demand Transfer Case — Eight Speed
Transmission Only
The 4WD HIGH and 4WD LOW positions are designed for
loose, slippery road surfaces only. Driving in the 4WD
HIGH and 4WD LOW positions on dry hard surfaced
roads may cause increased tire wear and damage to the
driveline components.
Proper operation of four-wheel drive vehicles depends on
tires of equal size, type and circumference on each wheel.
Any difference in tire size can cause damage to the
drivetrain.
Because four-wheel drive provides improved traction,
there is a tendency to exceed safe turning and stopping
speeds. Do not go faster than road conditions permit
ADDED: I have not found any listing in manual for how fast ref 4WD High. This is where the Common Sense Factor needs to be applied. 4WD High is great, but everything has limitations, and dependent upon Multiple Environmental and Operating factors, and
COMMON SENSE should be at top of list, IMHO.