To buy new tranny or rebuild old tranny, that is the question.

holy crap that's not new at all. mine makes a LOUD clunking noise under certain circumstances, primarily if slowing down on an uphill grade going from 8th to 7th, it drops into 7th HARD, it sounds like drive shaft lash that goes from the clutch to the dif, it gets confused by the deceleration because the road speed isn't where it thought it would be shooting for with the RPMs. mine also moves forward slightly from N to D, but not a lunge, it just pulls against the brake to probably set the gear in place. you don't pull liquid bulk do you? liquid bulk confuses the brains out of mine from the surge, it goes to shift expecting a certain road speed, the surge slows the truck down by a few MPH and then it's like "oh no i'm stalling" and drops 2 gears and bounces off the rev limiter. it's ALWAYS WHEN THERE ARE 6 people watching me crawl through a busy light. i basically never use auto mode anymore, it makes me look like a monkey fighting a spaceship.

 

I'm sure it is something simple, may not be cheap, but something easily fixed. Just be leary of where you take it... These shops out here are vultures, and we are carcasses to feed off of. Good luck.

 

I cannot answer your autoshift questions but I have been nearly stalled in an intersection due to slosh.
Comment on the exchange or rebuild I will.
At one time I worked at a bumper to bumper truck shop. A regular customer and long time O/O needed front brakes." Replace the drums while you are at it. Also pull the hubs and inspect the bearings". He came through while it was apart. "How do the bearings look?" Like brand new. Everything is cleaned up and we are getting ready to put it back together. "I sit behind that steering wheel a lot and my mind thanks about a lot of things. I have sat in that seat over those wheel bearings over a million miles. Just go ahead and replace them anyway so that is one less thing I thank about OK".
He also scheduled most major repairs and would take the family on vacation during that time. Pick the truck up and write a check for the bill.
If you are half way like him rebuild yours, you know what you have. When you get an exchange it could be from someone like you. It could also come from everyone else's junk when they had enough pieces and a slow day.
Hope it is something simple.
I have also been told just mash it to the floor. It will find something. That's what I do and it did. It was rough and loud.