Pre-purchase survey by a client who is thorough in his due diligence. He wants the stability testing before his sea trial.
I would think one of the "builders" in the area would be your best shot at knowing who is most expert in doing so, since they're going to be the ones with the facilities for it. Rybovich, Roscioli, ACY, Derecktor. I have no direct experience in South Florida. I do know World Yacht/Pliske says they do stability testing.
To answer the OP's original question, In Ft Lauderdale speak with either David Morrison or James Brewer @ Derecktors and they'll steer you in the right direction locally. Or you might try Jake @ the Marshall isl registry in Lauderdale.
I would call Boksa Marine. They are in the Tampa area but not too far of a drive. I have worked with them for many years.
Thanks for all of the responses and input. I received a PM that gave me 2 names not posted. I spoke with both and was impressed. If anyone should need the names, please send me a PM.