I would go ahead and hang on to MJD then. If it was a keeper league, I would almost suggest trading him (I have him in a keeper league) with his age to 30 coming up.
On a random note, I really like Michael Bush this year, even if he is just on your bench.
Calvin will likely have another great year, but top receiver again? Doubtful, that almost never happens. What he does have is Stafford though, who I love. Best yards passing in 30 starts except 2 other QBs in history. If he hits Calvin more, you could be in for a good trade there. You have two good receivers at the top. If you get Johnson, you're looking at also having a receiver in the flex spot or having one of them on the bench consistently. If you feel that Jennings and Brown are going to pull good enough seasons to be on par or a little under Calvin, just keep MJD. If you have doubts about one of them, go for the trade.