I'm back from a weekend vacation at the lake. Towed my aprox. #1200-#1400 boat with my Double Cab V6 4x4 Taco. Wondered about using 'D' and just now read a bunch of posts.
Here is what I did before reading up on it:
Drove in 'D' only if running 70mph to 78mph [trailer has tall tires and no sway]. Drove in '4' if under 70mph or in hilly areas.
I never use 'D' if it hunts more than only an occasional downshift when steep hill comes.
One thing I did not see in the posts that I read: what about engine oil pressure/oil film strength when your engine turns lower rpms AND is pulling a trailer....even if your trans does not 'hunt'? What is the lowest towing RPM recommended if towing and in 'D'?
I only used 'D' on the expressway here in Georgia and most is FLAT or only mild inclines.
Also, I don't like shifting between 'D' and '4' when at speed. It works ok but I 'feel' a little more mechanism 'working' than seems good. Best to decide what you are going to need to be in, and select your gear when going slow or at a stop?