system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does my truck manufacturer suggest not using 4WD over speeds of 55 MPH

I've been looking online for this and can't find a suitable explanation. It just says in the manual to only operate 4 Wheel Drive High under speeds of 55. Is that just a "cover our asses" recommendation or is there a legitimate reason?
gpt
When you're in standard drive, that is, with only power to your rear wheels, your front axles aren't powered and your transfer case is disengaged. Your front wheels are free to spin at whatever RPM they feel like. The RPMs of the front wheels will be different from your rear wheels in several situations, most noticeably when you're taking a turn because your front wheels travel further (especially further than the rear wheel on the same side as the direction you're turning).

Your transfer case does not have a differential. A differential would allow the two pairs of wheels to spin at different RPMs. Without it, the front and rear pair are forced to rotate at the same RPMs.

When turning, even gradually, you will feel shuddering and tire skipping because your drivetrain is fighting itself as one set of wheels is forced to rotate either faster or slower than optimal to match the RPMs of the other set. This increases wear on your drivetrain, and gives a bad ride quality.

edit: axles, not transaxle.