First time driving a truck in the rain. It felt a little loose in the rear so I put in 4A on the fly at around 55mph. It felt like the truck was weighted down and stabilized after putting it into 4A. Is it fine to use in the rain like I had in this video?

https://streamable.com/c4nmi

 

Yep. It doesn't weight anything down, but it does help on spinning the back tires on takeoff.

You'll find tidbits like this, plus many more, in the Owner's Guide located conveniently in your glove compartment.

 

I live in Florida where hard rain is the afternoon norm, I just leave my truck in 4A most of the time. It really does effect my MPG too much and also faster off the line

 

Check your owner's manual.

4-Auto is appropriate for ALL road conditions

Operating a hand held electronic device/phone while driving at 60 mph in the rain, less appropriate