Check and see if the transmission ECU is full of ATF.

Yeah, I checked it out, bone dry. Read some post on that problem.

Still leaning to a bad torque converter rebuild.

As MBDOC said, the P0715 code usually means a bad speed sensor which is an integral part of the conductor plate above the valve body. It's about a $170 DIY part plus fluids and gaskets and you need to reset the ETC codes with a Star Diagnose System or equivalent to get the trans out of limp home mode. An OBD-II scanner can't do that level of reset.

__________________
Reid
Past ('59 220Sb, 2-'59 190's, '92 300D 2.5T)
Current
'04 E320 4Matic Wagon
'00 E320 4Matic Wagon

I've seen this statement by many posters over the years. For the sake of accuracy, and without ruffling any feathers, I'd like to explain how it really is.

The transmission will come out of limp mode as soon as the condition that caused it is rectified (e.g. conductor plate replacement due to faulty speed sensor). The presence of stored fault codes will not affect operation. Current faults are a different story.