Horse is a poor downhill walker

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From: Barb Peck 
Email: bpeck@together.net

OK everyone, I need help with this one:

I have a horse that since the day I bought him
seems to have trouble walking downhills.
He can jog or trot downhills perfectly,
passing every one as he goes (and has never been chest or front end sore).
I've had other people ride him,
so I can watch 
 and it appears to be somewhere in the back end.
Another reason I think it's back there somewhere
is that when he's shod, he doesn't like
to have his hind legs lifted and pulled away
from his body (when the farrier does the clinches).

When he walks downhill
He takes itty-bitty steps with his hind legs, which really slows him down.
Conformation wise he's symetrical and his saddle fits fine (I'm a saddle fitter). He tracks up at the walk, and over at the trot. He's ridden bout 85 miles per month Apr. - Nov.
He's supple to the right and left.
He jumps without problems and his attitude is good. On really steep grades
he can sit down and slide, no problem.
He's not post legged, if anything he'd over angulated (in the rear). Although he does not have much hock action, 
my vet says his hocks are sound.

He doesn't toe out or have cow or sickle
hocks.
He just can't seem to WALK downhill at
a decent pace. (It's not for lack of practice, we have alot of hills)

Can he be too straight in the rear?
What is this?  Stifle?  I'm thinking of having a Chiropractor take a look at him... Good idea?

Please respond privately if this isn't
a topic everyone is interested in.

Thanks,
Barb