Transmission Problem --what am I lookin at here?

[jamesd553]

I have a 2002 X5 4.4 with 117k miles on it. The Last 20k have cost me probably $4,000 in repairs. Mostly cooling system and electronics. Now the transmission makes a whirling sound when moving forward especially when accelerating fast.

Took it to a independent mechanic here in Sherman Oaks, CA (He was recommended on Angie's list and Yelp.) He used a Snap-On Diagnostic computer and got a transmission fault code (48 something about Lock clutch too much slip) He says that its gonna probably need to be rebuilt. Replacing with a used one if you can find one is not going to be that much cheaper because of the labor involved.

Is there a mechanic that I should take it to in Southern Cal that would be able to fix it ? About how much should I be expecting to pay? And is it is definitely a problem in the transmission? Not a ECM or sensor problem?