Ok so I’ve got a 2000 blazer 4x4 (4 door) with 175k miles on it and am at a loss, I have the 4l60e trans and it hasn’t had any issues whatsoever until now. About 10-15k miles ago I dropped the pan and changed the filter along with both of the shift solenoids and put fresh ATF back in it (about 5 qts worth) and everything was great, shifted smooth as butter as it always has. well the other day, my idiot friend though it’d be cute to neautral drop it from about 2.5k rpms, well he ended up busting the spider gear in the rear end, so I made him pay to get a new rear end from a bone yard and me and him put it in. Rear end is good, and it’s the same 3.42 ratio that came out of the truck factory. but when I shift into D (4th gear) to drive like normal, nothing. Same thing if it’s shifted into 3. however, 1 will drive fine, then shift into 2 and so on, it’ll shift into 3rd and 4th if I’m driving at speed and shift it like you would a manual trans. but from a stop, 3rd and 4th acts like the same as neutral. what’s even stranger, is that if I put it in “2” from a start, it’ll move but very slowly as it’s acting like it’s actually starting in 2nd gear which it’s never done before. is 3rd and 4th gear shot? I don’t see how a neutral drop would affect 3rd and 4th gear since it starts in 1st gear anyway lol. I dropped the pan again yesterday to check to see if I saw anything and I didn’t, fluid was still nice and red looking, didn’t smell burnt. But there was some black/grey “soot” at the bottom of the pan, it didn’t look metallic but it was on the bottom of the filter and the bottom of the pan. Any suggestions? Or am I looking at a rebuild?

Classic input sprag failure. Transmission needs to come out and be rebuilt or replaced. Just remember the more you drive it in this condition other parts will be damaged that may raise cost of rebuild if that’s what you choose to do.