this doesn't really apply, but one time, my brother got a buick from his MIL. the body and interior was really clean, but the transmission [2004R] juice was absolutely BLACK and smelled like burnt muffins. i don't think the fluid and filter was ever changed. i put a shift kit in it, and a drain plug in the pan. after it was fired up, the transmission shifted great ! he drained the fluid and re-filled it about every 200 miles or so, until it was cherry red and stayed like that, then ran it another 500mi. then it was filter and fluid. i think he did that 5 or 6 times before it cleared up.
he then ran that car for years until someone offered him a bunch of benjamins for it. transmission acted like new when it went by, by.
point is, it may take you a couple of fluid changes to get the milkshake out, but if you can't get it to shift right with no kick down adjustment, or sliding into gears like mush, it's toast.
at this point, you really have nothing to loose.