When you say differential are you talking about the ring and pinion that sits between the steering clutches, or one of the final drives. I ask because a crawler has no spider gears, therefore no differential.
That said if your talking about the ring and pinion your gonna have to set the backlash between the gears regardless of wether you go with used pieces from one or multiple places, or if you use all brand new stuff. Either way it's not usually 'that big of a job' unless it's something your not used to doing and don't have the proper tools to do it.
If it's a final drive there isn't really anything like backlash, etc, etc that needs to be set/adjusted. So, again, multiple used parts or all new stuff, either way is going to be the same amount of work.