Occasionally when my dog runs he lifts one hind leg, which makes him
hop on the other leg.  IS this a problem/condition/disease? He does
not do this all the time, only occasionally when he runs or walks fast
and does not appear to be in any pain.  It almost seems like he is
moving so fast that this helps him keep up.

I have looked up that these dogs have Subluxation of the patella which
they say when it occurs "the kneecap (we're talking about the rear
legs) slips out of its groove ? sometimes often and sometimes rarely ?
depending on the severity of the problem. If your dog is one of the
unlucky few whose kneecaps slip often, surgery may be the solution. A
dog with a mild case can live a normal life, kind of like a person
with a trick knee. Subluxation of the patella is a relatively common
problem in small breeds and some large ones as well."

But most of the websites do not describe what the symptoms look like. 
Does this sound like a problem with my dog that I am blowing out of
proportion?

Please take your chihuahua to the vet and have him evaluated.  There
'could' be a problem or it might be nothing serious - like you
explained he might be trying to catch up with himself.  In either
case, we couldn't possibly diagnose over the internet.

Good luck to you!
tlspiegel

Amen, Tlspiegel! Testify!

Seeteekay, this is a ridiculously easy check for your vet to do.
Limping is the most common symptom of a patella problem but your
symptom is not unheard of.

Anecdotally, I have seen my share of miniature breeds-- never happens
in anything over about eight pounds-- that lift BOTH hind legs when
running. Flippin' hilarious to watch.

Either a quick visit now or when your doggy's annual visit is coming
due and you'll know in a minute if there is a problem. Best of luck!

markvmd -  my 3.14oz chihuahua (almost 4 mos) sometimes lifts both
hind legs when running.  He's so fast, and it IS like he's trying to
catch up with himself.  Very funny!  :)   He'll also lift both legs
when eating his dog chow, and I fully expect him to fall face forward
into his bowl.  LOL