840 CI Not The Guibo or Center Bearing

The droaning and slipping at 1500 RPM in fifth gear does not appear to be the flexible disc/guibo or center bearing according to a mechanic who says he services all foreign makes including bimmers. He wants to service the tranny to see if the filter is blocked or if there is gear damage. The droaning/slipping only seems to occur in fifth gear at 1500 rpm in Drive and at 2000 RPM in Drive Sport Mode. It really does seem to point at the transmission if it only occurs in fifth gear and at a specific rpm range.

Should a BMW with only 68,000 miles have tranny damage even if it was driven hard?

The transmission service will cost $170 plus $58 for the filter kit plus $297 for fluid. The transmission should be serviced but it seems an awfully high repair cost that may not solve the cause of the immediate problem. The car is very miserable to drive because the droaning/slipping problem occurs at normal cruising RPM, so it is constantly there.

Before I have him service the transmission, should I bring it to BMW and will they be able to diagnose the problem without guessing it is the transmission? I have searched this site for Transmission Issues and have seen that trouble codes are limited for diagnosing the tranny, but can this possibly be electronically diagnosed by a BMW Dealer?

Finally, could this problem be a blocked transmission filter, a rear end or a rear wheel bearing issue? My fear is that I have a bad fifth gear. If it is the valve bodies, can they be replaced or am I looking at a $6,000 rebuilt transmission?

Thanks much.