Troy Bilt Pony deck height

I have a Troy Bilt Pony.  Not sure exactly how old it is, but probably around 10-15 years.  It was purchased new and has never had any major work done on it.
 
Over time the deck seems to have dropped lower and lower.  At first I had the deck in the #3 position.  After some time I had to put it in #4.  A couple of years ago I had to start using #5.  Now the deck has started digging into the ground.  
 
When I look at it, it doesn't look like it can be lifted much higher.  It might be able to come up 1/2" before it hits the frame.
 
If I try to lower the deck, by the time I get to #2 it is on the ground.
 
What should I be looking for to correct this?  It seems almost like something has slipped or been bent, but I'm not seeing it.  Maybe I just don't know what to look for...?
 
I know it has a "floating" deck.  I've wondered if it is floating too much sometimes.  Our lawn is small but has a steep hill so I am often mowing across an incline.
 
Any advice would be greatly appreciated.
 
Thanks!