6l80 is dying I think
 

Never lost a transmission before but I think I'm about to. Recently when driving normally the truck has a very hard clunk of a shift into second gear. When driving aggressively it shifts fine. Is this a sign I'm about to loose the transmission?

Mine is in the shop being rebuilt now. I was in the drive thru getting some food and it started getting hot. Message popped up on the dash "transmission hot idle engine". Drove home it went into limp mode (1st and 2nd gear only). One week later 1-2 shift popped rpms went up and that was it. Couldnt get out of 1st gear. 150k miles on it. Everything was pretty much trashed inside the trans.

I think mine has done the same a few times when not warmed up yet but no issues so far.

I've heard that when they are on their way out they bang into gear because the adaptive shift just starts throwing everything it's got at the shift to make the shift happen at the speed it wants it to shift.

Hopefully that's not the case. If so I've talked to circle D and another very trusted trans shop and 6L's are minimum 3500 to rebuild stock. You can buy a new one for around 3000. I bought a high mileage used truck and was worried about the trans and started looking around. I was shocked at the price cause I was used to 4L60e's. So then I started looking at used ones. You can find 20k mile ones in junk yards for 1200-1800. Most yards are 1200-1300, LKQ's shit don't stink and they want 1800. I asked if they would compete with prices of other scrap yards and the arrogance from the dude was really aggravating, long story short they aren't coming off the price one penny even being that I called and used my shop account and we buy a lot of motors and transmissions from them.

If you get a used one, you need the 4 letter code off the side of the trans. It's something like ACCA or ABCA or something of that nature. If I remember right I found the code on the passenger side of the trans about half way up the side of the trans, and about half way from front to back.