1998 Jimmy SLS Towing Question

Hey all, my jimmy has the "towing" package (I lucked out on that when I bought it, tranny cooler and all) and is running on the automatic. I'm currently moving houses and I've got my hands on a nice trailer to load my couches and stuff up on. I know my load limits but here's the question:

The highway I take has lot's of hills on it, so I went to look in my owners manual to see what setting it recommends for the auto, alot of the trucks I've owned just have a "tow" button that switches out the OD. To my surprize the owners manual says "make sure to drive with your transmission selection in (D) to avoid damaging the unit". Correct me if I'm wrong but I always thought if you don't have a tow button to slip the OD out then you always put it down into 3 since the D option above it is the OD option. I drove the trailer home partway (empty of course) with it in D and it was a real pain in the ass on hills so I started using 3.

I would assume I have to use 3 on the highway with all the hills contrary to what the manual tells me to do. Does anyone know this for sure? The only reason I ask is that because "normally" the manuals are well written and correct.

Nothing? My gut says go with 3 but my manual advises to only use (D). Anyone? I'm leaving tonight so I guess either way I'll post up if I blew something.