There's a big difference between a "JAKE" brake and and exhaust brake. You can't put a JAKE brake on your Dodge, but you can put an exhaust brake on it. NO, an exhaust brake will not void your warranty. Do a search on JAKE BRAKE in the the forum and library. This topic has been hashed over several times.
Dodge outlawed exhaust brakes on their trucks for awhile - they were having problems with automatic transmission failures when an exhaust brake was installed. I believe the restriction has been lifted now, but check with Dodge on your 2004.