There are two issues here, both related to whether you use it as a
truck or as a car with a really big trunk.
The LT (light truck) series tires usually have a higher load range than
do the Passenger series. One would expect the tradeoffs to be higher
cost and lower carlike qualities such as handling and noise. If you
drive around with a bed full of construction materials or a heavy
camper and two weeks' worth of hunting and fishing stuff or whatever,
it might be worth your while to consider gross weight.
Either series can be had with a variety of tread styles from
highway-only to significantly off-roadey. When going on bad roads or
none at all, you usually want a blockier, more open tread that looks
like it'd dig into mud for traction and fling it out afterwards. Such
tires also tend to have tougher, straighter sidewalls so you don't kill
'em on sharp rocks and so forth.
Winter performance, if that matters to you, can be harder to tell just
by looking, though if going through deep snow matters to you, a blocky
tread sure helps (this from driving both an S-Blazer with mildly
SUV'ish passenger tires and a new Dakota that still had its OEM
all-terrains in snow last winter -- it made the difference between a
churny, squirmy excursion and just another day at the office). Check
out the ratings in addition to eyeballing them. Of course, if you deal
with serious winter a lot of the time, a set of steel rims and some
snow tires might be in order.
The bottom line on the bottom of your truck: yes, there is a
difference, but which way you want to go (to engineer is to compromise
or specialize) depends on what you mean to do.
Probably most people with pickups and SUVs are fine with a tire in the
passenger series and the highway style -- but there are also numerous
exceptions, and if you're one of the exceptions you wouldn't want to
get stuck (in more than one sense of the word) with a badly wrong tire.