I am new to trailering, sort of.  I recently bought a 2002 Ford F150 and a 2 horse bumper pull (older trailer, steel).  I am used to driving a friend's SUV and her 2 horse trailer.  She always had us drive it in 4 wheel drive.  the dealer that i bought my truck from said to never put it in 4 wheel when i pulling the trailer loaded or unloaded.  I only truck one horse at a time by the way.  he said that the only time i would need to put it in 4 wheel is if it snowy or muddy and he also said to NEVER put it in 4 wheel low.  he said the only time that would ever be needed would be if, say, i were pulling a boat out of the water which i won't ever do!  I just always thought that it should be in 4 wheel whenever it's being used to tow something...but then again....what do I know!  THANK YOU!!!

I am unfamiliar with Ford but in the half ton GM trucks there is 2HI, Neutral,  AWD (All Wheel Drive), 4HI, 4LOW.  Towing in AWD is okay.  AWD will send traction to the front axle if slip is detected on the rear axle but the transfer case is never locked up like it is in 4HI or LOW meaning that the front axle and rear axle are locked together.  Towing on the road  in 4HI or LOW is very bad for the transfer case and will cause great expense in a short amount of time.

When your truck is in 4WD, the front and rear drive shafts turn at the same ratio and are locked together. This is fine when the tires can slip, such as in mud or snow.

When a vehicle rounds a corner, each wheel travels at different speeds and distances. This is the reason a differential is necessary, to allow the wheels on one axle to travel independently of the other.

When the axles are geared to turn the same ratio, when 4WD is selected, the front and rear wheels are forced to turn the same amount. If they can't slip as when they are on concrete, the whole gear train will be subject to over loading, and damage can occur to many of the components.

As was stated, the only time 4WD should be selected is upon slippery conditions, and not on open dry roads. Vehicles that are designated as all wheel drive, have various coupling techniques, that allow the wheels to lock up when they are needed. They are electronically controlled and different from a standard part time 4 WD truck system.

We had a 76 Ford truck with a full time 4WD system. It had a third differential in the transfer case, and used another couple of miles per gallon just to go down the road.

As gard said. If you have an owner's manual, it will tell you in there to ONLY use 4 WD when it is slippery.  I have owned a 4, F150s - 3 of which had 4WD.

I have used the 4 WD when towing when there was snow, wet gravel, wet grass on a slope... and the other time is up a VERY steep somewhat loose gravel road that I have to start from about 5 mph - I feel that the added front wheel drive pulling the truck & trailer is more benefit than not.

I am wondering if your friend has OVERDRIVE confused with 4 WD.  You will want to select Overdrive OFF when towing with the F150. That will prevent the transmission from shifting back and forth so much. Your RPMs will run a little higher with OD off and will take more gas, but the cost savings on your transmission is well worth it! 

I can't remember where the "switch" is for turning OD on and off on the 2002 - on my 2006, it's a button on the END of the gear lever.