How to Find a Model Number
I'm not getting any hits for your model number. Please check it carefully and repost it.Have you tried getting error code(s) from your machine? There's a booklet inside the cabinet at the top left that will explain how to put washer in service mode and retrieve error codes. You'll have to lift up the entire top to get the booklet.
Check the W10734521 slider. It should have sharp edged teeth that engage with the rotor. Check the rotor too to see that the teeth are sharp where the slider engages it.
Does slider move freely? It should be up or down but not in between.
I've seen number of Whirlpool washers of your design that have prematurely worn slider, rotor, and other parts so the design is not robust. All parts that we've discussed so far should move freely and easily with no prying or tapping to move. Slider should be all the up or all the way down and nowhere in between. If you run a normal cycle now, what error code(s) are stored?
Got any updates for us? Did you replace the slider?