Not my first broken mike, but what's remarkable is that this one's a presumably indestructible SM-58 less than 6 months old.
 
It suddenly started sounding thin and weak, like a 5-dollar Wal-Mart special. It's never been abused. Not even dropped, much less put into a microwave oven or used as a hammer.
 
So I'm off to the music store this morning to buy another one.
 
Anyone ever had a 58 go bad? Anyone test Shure's 1-year warranty?

Bad luck aout your SM58. They have a reputation of being indestructible. I have a twenty year old SM58 which still works but I think it has lost some of its 'edge' over the years. For studio use, I now only use Rode mics (NT1A, M1, M2 and M3) not only because they sound great but they come with a ten year warranty. A company will only offer a ten year warranty if they have faith that their products will go the distance.  

Maybe a connection problem with the XLR jack..?
 
http://www.harmonycentral.com/forum/forum/LivePerformanceCategory/acapella-33/387137-
 
If you get a warranty hassle, the above post might offer some ideas worth pursuing.

I have an SM58 that I bought in 1980. Saw a lot of gigs.
Still works just like new.
I used it a couple of months ago to hammer in some fence posts out back.
Just kidding....I did use it recently though. It performed just fine.
Are they slapping them together in an overseas sweat shop now?
(Some fake knockoffs out there these days to watch out for as well.) 

My first mic, bought at age 15, was an SM57. It cost $40. For the next 50 years I never once heard of a Shure dynamic breaking. I used that one for 10 years before it got retired, not because it failed but because I discovered that AKGs suited my voice better.
 
So I was reluctant to accept that this new 58 was busted, but it most definitely is. I bought another one yesterday and used it at band rehearsal, and it puts out twice the volume - same cable and PA.
 
Turns out, the 58 has not a 1- but a 2-year warranty. I will therefore wrap up the broken one in the box the new one came in and ship it off to Shure.  I'll report back how that goes.
 
Amicus: thank you for that link. The problem described by the OP in that thread sounds exactly like what I'm experiencing. I'll definitely inspect the internal XLR connections before I ship the mic off.

My good friend and band mate for over 30 years has 3 SM57's and 3 SM 58's he bought in the mid 70's SECOND HAND. He still even has the original sacks and cables that were included back then. 
To the best of his knowledge those cables have never been repaired!  And these mikes and cables have seem it all. Drunk band-mates tearing down ( sober on set up) stomped on, rained on, beered on,  dropped on concrete the works! They don't look pretty.  
A couple of years ago we A-B'd them with my newer versions after I cleaned the screens for him. ( gross)  We also ordered some new windscreens for 2 of the 58's. 
We could not find any difference in ouput level using a test tone at a measured distance. 
I also tested the full spectrum into Wave Lab using pink noise(?) any how we found no difference between the old and the new so they still live on. 
 
My SM 57's were bought in the 90's and made in Mexico. Three years ago I bought three SM 58's on a Musicians Friend stupid deal for $80 each.. I think they say Made in China. So that might explain why your mike was faulty. It can be expected with any trusted brand that in the name of profit off sources to China..
There's a small pre amp (?) inside the mike and very tiny wires running from the capsule  to the output connection, Never on a Shure, but I've had those come loose on other brands. 
 
I myself have used my Beta 58 for just about everything I do, live and in the studio. I gave up on trying Large cap condensers. I make to much noise :)  I use those for other people.