My truck runs perfect in 4x4, but when it's 2wd the transmission makes a loud ticking noise! Makes the noise in all gears and nuteral. It's louder in 3rd gear then the other gears

 

You are talking about transmission, so would it not be important to mention whether you have auto or stick shift?
A problem well-stated is a problem-almost solved.

Kinda sounds like my symptoms. Does sound change when you press on clutch pedal?