on my 03 3500 truck the transfer case oil seal leaks how do i change it.

 

Block the wheels.
Trans in neutral.
Can jack up the rear if you like, makes life a little easier.
Drop the driveshaft.
Remove old seal.
Clean surfaces.
Install new seal.
Put driveshaft back in.

Not to much of an ordeal. Hour or so worth of work.

Your truck is 3rd gen though, so, should be posting in that section.

Welcome to DF!