The others have made good points about cyclocross, you probably don't want to risk your shoulder with that at this point.

Why not wait a bit before selling your mountain bike? You already have it and you won't be able to get your original price out of it...and should you decide next season you would like to try it then you may regret selling it and could well pay more to replace it. I do know people with shoulder problems who are able to mountain bike without pain.

Another option is to setup the mountain bike for other use. I know people who use them as commuters. They put skinny tires on them and go for it.

I certainly understand wanting another bike, been there a time or two in the past two years Good luck with whatever you decide!