Honestly its not really the strain on the engine, but rather the strain on the transmission. Are there mountains or steep hills where your going?
I have a 2005 1500 chevy, so the same truck, and have been hauling horse trailers for about 3.5 years with it and haven't had any problems (knock on wood). My mom has the same trailer that you do, and I have hauled 2 horses with her trailer for about 2 hour trips. I have hauled 1 horse for 5+ hours with my trailer, but haven't really done 2, but wouldn't hesitate if needed.

I will say that the gooseneck hauls WAY easier. You can tell that its easier on the truck to haul the 3 horses in my gooseneck (never for more then 1/2 hour) then it is to haul 2 with the other trailer.

Do you have anti sway bars installed on your hitch? That will help stabilize it and make it easier on your truck. If you do need to do it, just drive really carefully, don't accelerate going up hills and just keep your foot steady and allow your trailer to slow if needed. One thing that helps with the truck is to keep it in 3rd gear instead of drive, that will keep the transmission from kicking into over drive and is easier on the engine and transmission.

I feel your pain though! I have a 3 horse and unless we are going less then 15 minutes my hubby kicks a fit about hauling 3, even when one is a 13h pony! I do try to bow to his greater wisdom since he has been hauling and working on trucks since he was 16 (now 32).

We eventually want to get a bigger truck, but can't financially swing it right now.
(my truck and trailer)

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