Transmission

2001 Chevrolet Silverado 3500HD Duramax 6.6 liter turbo-diesel w/Allison transmission 4X4 dually - 127,063 miles. Bought used in March of 2004 with 80,000 miles. When purchased, the engine slightly reved in rhythm while idle and still does. Took it to Chevrolet dealership early on with this sound and they wouldn't take it in the back because he said, "That's the sound diesels make". It is my understanding now that this is b.S. Anyhow, this year the sound has gotten louder - but not obnoxious. I took it to a reputable transmission shop - not for that reason - but they noticed the sound and diagnosed, tested and replaced the speed and turbine sensors. I'm telling you all that, just to give you everything I know. I don't know if it relates to anything I'm ABOUT to tell you. BTW - the truck has no leaks that I'm aware of. Okay. When starting the truck, after being off for a long period of time. Enough for the engine to be cooled down, it takes about 1/4 to 1/2 mile for the gears to want to change - at this point, I'm only doing about 30mph - tops - and then all is good from then on. I live in southeast Texas - so, it's nearly never cold here - just FYI. I am a 37 yo female, and what I know about automobiles - other than general maintenance, could fill a Post-It note.
The major problem is: First week in October, after driving it hard for 30 miles to an ER in Houston traffic (gunning it and slowing) we got to the parking garage - while slowly entering the garage, I lost ability to go forward while in drive - except flooring it helped me go about 2 feet. I put it in 3rd and it finally moved okay. I parked and after 2 hours, it was fine to drive home - until after slowing to pull back into my driveway - that's where it started again - had to put it in 3rd again. For the next two days. It did it again after being driven for about an hour or three. Took it to the reputable transmission shop, as stated above, and they kept it - per my wishes - for a week to try to get the problem to replicate - with no luck. They changed the two sensors and charged me $257.00. And more than slightly decreased the reving noise. Now, one month later - the reving noise is back to it's tolerable, but still not obnoxious(but still louder than last year) level. Today, the truck started again with the "not wanting to move forward after slowing down" thing and I had to put it in 3rd again. I limped it home for 1 mile in 3rd and then 3rd didn't want go after stopping by my driveway. I put it in park and then when I put it in drive again - something under the truck slammed "KaChunk!" - Really loud - this has never EVER happened before. Tried Drive again. No go, tried reverse - nothing! - It wouldn't move - put it in 3rd and it went again! What the Hell! (Madness setting in.)
Parked the beast on the street and turned it off. Two minutes later, had to move it up a bit - put it in drive - NO KaChunk this time - but had to move it in 3rd. Two hours later, moved it up a couple of feet to see what would happen - no KaChunk, no problem in drive or reverse. Like nothing happened! I love my truck - I would sell my husband to a band of gypsies to keep it! Please, will you/can you explain to me what's going on and how much do you suspect it's going to cost to repair it? - And ANYTHING else you all think might be pertinent. Thank you so very much for your time in reading this. I hope desperately that someone can help, so that I might not get screwed to the wall at the shop. :Shock: