Hope you Feel Better, SOON, Elvis!

Sounds very similar to what Smokey has -
a Luxating Patella or sometimes called a "trick knee"...

Smoke's left hind Knee Cap is "loose" and slips out of it's proper
position - which sometimes causes quite a bit of pain when
it "goes out"!

A Luxating Patella can usually be diagnosed by an experienced Vet
just by "palpating" (feeling and manipulating) the joint -
(X-rays don't really show much for this problem).

If this IS what Elvis suffers from -
it can usually be surgically repaired. Only down side to the
surgery is the LONG "recovery period" - 4-6 months -
during which the dog must be restricted from running and jumping!

Smokey has "learned" how to stretch and *shake* his leg to
*pop* the knee cap back into place by himself!
If it really gets *STUCK* - he'll *yelp* and limp over to me
for Dad to help pop it back into place.
Ask the Vet to SHOW you how to assist with repositioning it.

And there could be a LOT of other possibilities -
ligament problems, hip problem, sore foot...
Sounds like you're taking him to some Expierenced Vets -
they should be able to locate his problem.

Good Luck!!

/s/ Phred
..