Hi, I'm having my 2001 DRV 3500 towed into a tranny shop tomorrow and ask if anyone can answer if the following sounds like the torque converter is shot;
Traveling thru Smokey Mountains started ok- I would come down steep grades at 75 to top next crest at 55. This was all in OD with no real strain on engine. Over the next few hundred miles this gradually changed to going down steep grades at 60 because I couldn't go faster without van downshifting (??) and then barely able top next crest at 20 with OD off and in 1st gear. Both up and down grades was putting strain on engine. Occasionally the van would "shudder" and at times of peek strain up grades I could hear a rattling or chattering noise. By the time I got out of the mountains on fairly flat ground the van would not go over 52 under great engine strain as if I tried faster it would downshift.
Concurrently with this and also gradually the van would start fine but conk out in D or R to the point now it instantly conks out in any gear except P or N.
Obviously I'm better off having an idea what my problem is before taking it to a shop. I had a flush and filter change and ran fine for at least 1500 mi before any of this and tranny fluid level is still good and not discolored.
Any ideas would be greatly appreciated.

5.9 with 72,000 mi. It sat for a long time. Whatever it is it happened in a 2 day period gradually driving real steep grades. Going down these steep grades in OD I set cruise control and never needed to break but van is 10' high top with lot of wind resistance. I'll post the diagnosis by Thurs.
Thanks for responding
Forgot to mention no OBDII fault codes register.