Tow rig trans: E40OD. Self rebuild or?

There's quite a knowledgable group here, so thought I'd throw this out:

Got a 1994 E350 with only 83K miles on it- 7.3L non-turbo diesel. I keep getting trouble codes: FIPL switch and Excessive Torque Converter Slip. I replaced the FIPL with a motorcraft part and set it within the recommended volts. no codes for a while (and by that I mean for the next 2 drives) but then it tripped again-- same two codes and it gets sent into safe mode-- full pressure shifts.

From what I've read there's a valve that cracks that causes torque converter slip. I've never ever ever seen the inside of an Automatic trans (well, maybe once) but I have changed a complete transmission on a isuzu npr box truck, and I can rebuild a mini's manual transmission, I've discovered.

That said, this commonly cracked valve thing-- is it a drop the pan and replace the part sort of thing? and could I likely get by by just replacing the torque converter? Or should I attempt a full rebuild myself? Or I guess I could kiss the next 3 Lemons races I want to do good-bye (on the 2011 calendar, of course) and use that entry money to have another tranny professionally installed or rebuilt?

Tranny shows no obvious signs of slipping. However, if I'm looking for it the rpms do come down slightly (100-200rpms or so) after cresting a hill for maintaining the same speed (if I watch the tach and speedo like a hawk). Rev sensor seems fine-- no jumping around and I pulled, cleaned and reinstalled it just because.

Thanks for any pointers.