Yes, there can be a difference between "avoiding" and "ducking" even though most of the time, the terms go hand and hand.
Avoiding is when there is a fighter out there who deserves a title shot but the champion (or best fighter in the division) will not give one to him. Many times, the avoided fighter will either not be a big name or will not bring that much revenue to the table, so the champion will choose not to fight him despite the fact that he probably deserves the opportunity more than any other contender out there. The majority of the public will not mind the fact that the big name fighter is avoiding the other fighter because there are bigger and more captivating matchups to be made or there are other factors as to why the matchup cannot happen (such as rival promoters, timing, circumstance, etc.), so the big name fighter gets a sort of pass from the public for choosing not to face the avoided fighter.
Some examples of fighters being avoided by big-name fighters are:
~ Charley Burley- was avoided by Sugar Ray Robinson and Robinson was given a pass because he had just moved up to middleweight when Burley retired and there were bigger matchups that the public wanted to see Robinson in.
~ Aaron Pryor- was avoided by Sugar Ray Leonard and Leonard was given a pass because there were bigger matchups (i.e. Thomas Hearns) and Leonard unexpectedly was forced to retire early due to a detached retina.
~ Juan Manuel Marquez- was avoided by Naseem Hamed and Hamed was given a pass because Marquez was an unknown commodity at the time and there were bigger matchups for Hamed.
~ John Ruiz- was avoided by Lennox Lewis and Lewis was given a pass because there were bigger matchups and everyone hated Ruiz due to his boring style. No one will EVER get criticized for avoiding John Ruiz.
~ Antonio Margarito- was avoided by Floyd Mayweather (NOTE: this was back in 2006) and Mayweather was given a pass because he took on the linear champion, Carlos Baldomir instead.
~ Miguel Cotto- was avoided by Floyd Mayweather (NOTE: this was back in 2007) and Mayweather was given a pass because there were bigger matchups (i.e. Oscar De La Hoya and Ricky Hatton) and Mayweather unexpectedly retired after taking those bigger matchups.
~ Celestino Caballero- is avoided by; Juan Manuel Lopez and Yuriorkis Gamboa and both are given passes because there are other matchups that are just as intriguing in the featherweight division although when it's all said and done, if neither ever faces him and and if he keeps winning, then Caballero will be considered a ducked fighter.
~ Paul Williams- was/is avoided by; Miguel Cotto, Floyd Mayweather, and Manny Pacquiao and all 3 are given passes because the public wants to see them face each other instead.
Ducking is when a champion (or top fighter) chooses not to face another fighter who deserves the opportunity to face him. The difference between avoiding and ducking is that in the case of ducking, the entire public wants to see the fight happen and it is unquestionably the biggest fight that can be made for either fighter at that particular moment and there is no logical reason as to why it should not happen except of course that one of the fighters is ducking the other. The fighter who does not want the fight is denying the fans the fight that they want to see while at the same time, not granting the deserved opportunity to the ducked fighter.
Some examples of fighters being ducked by big-name guys are:
~ Lennox Lewis- was ducked by Riddick Bowe and Bowe was not given a pass because Lewis was the top guy out there who had previously knocked Bowe out in the amateurs and Bowe had plenty of time to defend the title against him before taking on Evander Holyfield in a rematch but instead, he threw his WBC belt in the trash and told Lewis that if he wanted, he knew where to find it.
~ Wladimir Klitschko- is being ducked by David Haye. It's an odd case because Klitschko is seen as the true champion yet he is the one who is being ducked. Haye is not given a pass because a Klitschko-Haye bout is the biggest match that can possibly be made in the heavyweight division and despite Haye talking all of this trash about the Klitschko brothers, he gets fights with BOTH of them handed to him on a silver platter and then backs out of both fights to take on someone else. Then, he gets the opportunity to fight Wladimir again and once again chooses not to give the public the fight that they want and instead goes on to talk about facing an undeserving fighter like Audley Harrison. It's one thing to not take a fight for whatever reason but it's another to talk trash and degrade another fighter, get the opportunity to fight them and then go running off in another direction.
I do not have as many examples of ducked fighters because it takes a lot for me to say that one fighter ducked another because the majority of the time, there are many reasons why fights do not happen such as; circumstance, timing, availability, money involved, etc. But when you have a fight handed to you that you've been asking for and you have it handed to you on multiple occasions and you turn it down to face fringe contenders, there is nothing else that you can call it except ducking. I know you like David Haye because you used to have him as an avatar but I'm just telling like it is- I really hope that he does face Klitschko because I think that he's got the best chance at beating him outside of Vitali but right now, he's ducking Wlad and there's no way around saying it.
EDIT: Hector Camacho fought Ray Mancini and Marvin Hagler fought in a completely different era than Carlos Monzon.