Dealer is correct, unless you have original documents for the truck you won't know what sound system you have as far as the head unit(in dash)
Some have the amplifier in the head unit other one has a separate amp, usually in the passenger kick panel.
Have a look here: Ford Ranger Radio Wiring Diagram
Looks like 2005 had 3 radio options
Radio with amp build in for 4 speakers
Radio with separate amp for 4 speakers and subwoofer
Radio with amp built in for 4 speakers and separate amp for subwoofer
But yes to your question, if you swap amplified or unamplified head unit it would be plug and play, as long as they are both the same