How do you remove Large Decals [advertising size] from my pickup?

RickJ

Use a firm PLASTIC putty knife to carefully get the plastic itself off, then use something like GoofOff, or even alcohol, to remove the sticky residue.

WARNING: You may be left with a "negative" of the decals on your truck from sunfade - that is, from the general fading that you get from being exposed to the sun...but the part under the decal being a tad brighter.

...stay tuned, though, there may be others who will pipe in.

TxGreaseMonkey

WD-40 will also help remove the sticky residue.

kitch428

Hair drier. Warm it up first b4 scraping. Not too hot tho. You will probably just end up peeling it off.

Vandy-1

Yes, that is correct, this is what decal manufacturers advise you to do.

Fr_Chuck

A steam cleaner or a hot water pressure washer with a 15 degree tip also.

This is what the larger truck firms use to take decals off of the tractor trailers before they sell them