It's likely that you're right and that he was feeling unwell for some time before his amputation, and that damped down his behaviour.  He's now becoming the dog he would have become!  But don't worry, you can fix it.

It doesn't matter that you didn't do any training or correction while he was unwell, and didn't need those things, but you really need to begin to work with him now.  You don't need to do any of the old-fashioned, heavy 'Be The Alpha!!' training, but you do need to understand his behaviour and let him know in a way that he can understand that you will not allow it to continue.  Consistency, positive reinforcement and trading up seem to be needed here. 

Trading up is very, very useful for dogs who run off with things and won't give them back, especially when there's aggression (growling or snapping) involved.  What you do is keep a supply of really desirable treats on hand, and when he takes something he shouldn't have, let him see that you have one for him, but don't let him have it until he drops the forbidden object.  You combine it with training, because when you show him the treat, you will be saying 'drop it', and then 'good boy' when he complies, and you hand over the treat. 

It's a good idea to start off by practising this in an unthreatening situation, since you have been chasing him down to recover things.  If he plays with toys, start a game with a ball or something, and when he's holding it but not actively involved with shaking it etc, show him a treat, and say 'Drop it'.  If he drops the toy, he gets the treat. If he doesn't drop it, he doesn't get the treat and you walk away and try later.  Don't make a big thing of it, the aim is to make it his decision, if you can!

I like Patricia McConnell and Suzanne Clothier for easy-to-read books on dog behaviour and how they learn. 

Murphy is definitely a hard headed little beast. He has always been that way. The puppy trainer laughingly said that you train a Chesapeake with love and a two by four. We did tend to let things slide with him when he was recuperating, but the basics were still there. The one thing I have noticed is that for some reason he is much more protective of me.

After I had my recent surgery, I was sitting on the couch in the living room. Garry leaned over me to say goodbye when he was going out, and Murphy actually growled at him. He never did anything other than that, but it was pretty clear that nobody, even Dad, was getting near his Mom while she wasn't feeling well.

He has also become much more vocal about letting us know he has to go outside. Before his accident, he would just lay down by the back door. Now, if he wants out, there is a loud, insistent bark.

Kathi and Murphy