OK, but what specifically inside a "bad tranny" would account for the symptoms I described?

Our Kitchenaid KAWS977EQ0 washer is grinding during agitation. Using the built-in test modes, the washer will fill, drain, agitate at all three speeds with no water or clothes, spin normally, and brake normally when done.

However, when filled with water, the agitator makes a slight grinding sound during low speed agitation; the tub does not spin (which is good), but the lower agitator moves forcefully clockwise, then grinds a bit as it returns (but it does return); the upper agitator behaves normally, rotating only one way. If clothes are added, they circulate top to bottom and stay pretty much in the same area of the tub as they should. So for the most part low speed agitation is okay.

At medium speed, the agitator makes a loud grinding sound, the lower agitator moves forcefully clockwise, but grinds loudly when it should return, so it moves clockwise continually, making a turn-grind-turn-grind-turn-grind sound; the upper agitator moves by fits and starts since the lower agitator never returns properly. The tub does not spin (good). The water whirls around clockwise (bad). If clothes are present, they quickly wrap around the agitator.

At high speed, the agitator does all that, but much worse.

Initially, I thought agitator dogs. Indeed 3 of the 4 were broken, but replacing them merely improved the action of the upper agitator. The grinding noise and the whirling clothes remained.

Then I suspected maybe a broken motor-to-gearcase coupler, but the existing coupler had been replaced in the last few years and was in great shape.

Next, I found a broken cam arm in the brake assembly, and replaced it without effect. Then I replaced the entire clutch, with no effect.

Then I opened the gearcase and replaced the spring though it appeared fine. Replacing the spring had solved a similar problem with another washer years ago, but in that case the original spring had broken. None of these things made any difference.

I am requesting help in diagnosing this problem and in selecting a path forward.

I could replace the gearcase, but if someone can offer some guidance I would rather fix this one. When the gearcase was out of the unit, it spun and agitated fine, just as it does when installed and runs with no load. So it seems mostly okay and I would hate to send it to the landfill when maybe all it needs is one or two parts replaced.

The spin section of the gearcase seems fine, so options seem to be:

1. I assembled something wrong, but of course the whole reason for disassembly was the initial problem which is unaffected, so maybe
it is assembled correctly but something is worn/broken/missing. No debris was in the bottom of the gearcase oil when I drained it.

2. New agitate cams (#13 and 14 on the exploded gearcase diagram).

3. New thrust bearing (#17 and 18) - could affect the overall height of the drive shaft. The existing bearing has a tiny dimple due to wear but nothing more.

4. New agitate gears and washer (#11, 15, and 29) - teeth could be worn on #11, or the splines inside #11 could be damaged or the mating splines on #15 could be damaged, or maybe they are just both gunked up (should have cleaned 'em when I had it open). I suspect something is keeping these two parts misaligned when the washer is under load.

5. Replace the entire agitate shaft (#7). Kind of a shotgun approach, but if several worn parts are contributing to misalignment, this could fix it.

6. Replace the entire gearcase (#1). But I would hate to trash the old, nearly-working one, and the washer is my only testbench for checking repairs under load.

7. Replace the entire washer - not too desirable given the performance of today's washers.

I would really appreciate any help or ideas on how to proceed with this.

Can You Verify That The Agitator Is Actually Moving Back & Forth?