What engines would fit 03 Ram 1500?

Will be stopping by the wreckers tomorrow to see about prices on a replacement rear axle, if they have one, just to see how much it might be. Will be getting some quotes at some shops about fixing up the leaky rear pinion seal and most likely the bearing as well, since it is no doubt been damaged by the loss of fluid or at least ups and downs in the levels, for I don't know how long this has been going on before I got the truck.

While at the wreckers was going to ask about prices on any engines they may have that would fit into my truck, since it is almost 300,000 km on it. Just wondering what kind of engines would 'fit right in' and what years would be compatible with it. Its a 2003 Ram 1500 4x4 quad cab with a 4.7 V8 Magnum. Thinking there may be a newer engine from a wrecked newer model or something that would slip in, that had less mileage on it.

Other than a possible coolant leaking into the engine someplace and getting blown out when first accelerating (only scenario can think of as to why the coolant is always dropping with no signs of leakage and there is a big cloud of steam out the exhaust when first accelerating onto the street)this motor is really not acting up, just worried about its age is all.

Any pointers appreciated.