I think you've found a fairly normal case where this takes place - not necessarily indicative of a failure; e.g. a 'corner case' in an inherently weak design. This design weakness is in fact (in part, at least) the motive behind the infamous F37 reprogramming of the ECM.
Since you tow, an improved torque converter, and GDE's tune would clear the problem for you.
I've heard some discussion of a pump that can be improved as well but I think that the TC alone (SunCoast, for example) seems to give you sufficient torque handling where the shudder stops in the more extreme cases.
I'm sure there's plenty of guidance from the folks here who've actually exercised the various options on their own rigs. Mine remains stock thus far, though at 150k probably won't remain so 'forever'...
