I know this will be long but.. I'll add into this as well, even though others have said it. Using 4wd does nothing to help with braking. Using 4wd on dry pavement is hard on the transfer case,Diffs, drive-line components(u-joints). Mostly when turning, as the wheels are now all going different speeds. It cause the systems to bind up. You can even feel the truck start to 'twist' the chassis in the turns. I've seen folks blow there transfer cases forgetting to turn 4wd off. Yes you can engage 4wd on gravel surfaces, this is what I will do with my 2006 Chevy 2500HD with the Duramax Diesel so 4wd at-least gets some use. 4wd low puts a TON of torque to the wheels in order to help you get out or get going. Plus low top speed 15mph or so(depends on gearing/type of truck) In my truck 4wd Low I have to hold the brakes harder to keep it stopped cause it has so much torque. You can over-speed the engine really quick, honestly I wouldn't use it for towing on dry roads it's not going to help you brake.
IDK, what year your truck is or in configuration(ex cab, short bed, gearing?) how big or heavy your trailer is or how steep the grades are in your area. One this is, have you weighed your truck an trailer? or if not, have a place to take it an get it weighted? You might be over the max what your truck can pull and even over the max payload weight for your truck can be. It's very easy to do, especially with light duty trucks like an F150. In this case you gotta slow way, way down before approaching the downgrade. Make sure the trailer brakes are in good shape AND your trailer brake controller is adjusted properly to the weight of what your hauling.
Find out what the total weight is of what your hauling, even without a scale you can estimate the weight by finding out what the empty weight of the trailer is from the manufacture. What your hauling good sized horse can be an easy 900-1,100 plus pounds each, etc. then figure out the weight of the other stuff and go from there.
I've been down steep grades with my truck without issue mostly thanks to the help of the transmissions "grade braking" feature when you put the trans in "tow/haul" mode. It downshifts the trans and simulates what a manual does when you down shift earlier and uses engine compression to slow you. It works really well, I wish all trucks had this feature.