On my 2004 ram pu, I have tires size P275/55R20. Will tire size LT275/70R18 fit this vehicle?

Follow the tire size as required on the sticker on the driver's door jamb. Do not deviate from this, as there are many negative factors as a result. So - no, this will not works safely.

DEPENDING ON YOUR SUSPENSION, SHOULDN'T BE A PROBLEM. THE 18" TIRE WILL BE 1.3 INCHES TALLER, SO YOU JUST HAVE TO HAVE THE CLEARANCE. USUALLY A LEVELING KIT WILL PROVIDE THAT. DO NOT USE WHEEL SPACERS THOUGH.