One of the three shaving wheels rotates very slowly

could be bent axle but I would lean toward a bad tire. jack rear of vehicle qnd rotate tire slowly , if bearing is bad you will feel it. hold top and bottom of tire and rock, if bearing is bad you will have play or movement. rotate tire slowly and observe where is wobble is.

check the signal at the sensor with a multimeter. Most have three wires, one is a 12volt feed, one is a ground and the third is the signal wire. This wire is usually only 5volts and is grounded and ungrounded by the sensor in precise timing intervals as the sensor rotates. This means that to check its operation, make sure it has the 12volt feed and the ground, then, Jack up one rear wheel and chock the rest. Select neutral and release the parking brake.
Turn the ignition on BUT DONT START THE ENGINE. Reconnect ALL the wires to the sensor and then backprobe the signal wire you found earlier with a multimeter. Once you have the wire probed, slowly rotate the wheel by hand. The meter should show the 5 volt drop to nothing, then back to 5volts, then nothing, then 5volts and on and on as long as you rotate the wheel. If this 5volt is not present, check the instrument cluster as this 5volts comes from the speedo head. If the 12 volts is not present, check all fuses WITH A TEST PROBE - looking at the fuse is not enough and check the wiring to the sensor. If the ground is missing, check the earth point on the chassis under the rear of the cab and under the bonnet and repair and issues found.If all these tests show good and you have the alternating 5volt voltage, check the same signal wire at the cluster, if it still alternates when you turn the wheel by hand, repair the speedo head. If the 5volt signal feed from the speedo head is missing, check the wiring to the speedo head, if the signal is not present at the speedo head, repair the speedo head.

Sounds like you have stripped splines, damaged CV joint or popped axle retaining clip. jack one wheel at time, run slowly in drive with brakes off. the offending wheel will not rotate but axle will. if gearbox/diff axle should not rotate.

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That's a bad wheel bearing. If when driving and you swerve side to side and its loud one way and not the other or not as bad the other then its a wheel bearing. As for the metal shavings on the plug don't worry about it unless there are chunks of metal then you should have the rear end inspected.

Have you checked/greased the U-joints? Prop up all 4 wheels, put in drive and slowly rotate the wheels, and observe anything that dramatically slows down.

These may not be your problems since it is occurring even at low speed but you might have the car just free of the ground and rotate each wheel and watch the clearance while you do it for changing clearance. This will reveal 'out-of-round' tires; it happens. Second; but less likely, I had this annoying problem of two speed ranges, around the 30 MPH and 70 MPH points causing wheel shake and it only stopped after I tossed the original brake rotors and replaced them with Brembos.
In 50+ years of driving, poor tires have happened several times and brake rotors twice. There was a feature of a now-defunct chain called 4-Day Tires that kept me going back called 'tire matching.' All replaced tires were checked and then shaved slightly to correct for roundness. With this done, I never had any loss of balance during the life of the tires. Haven't found this offered anywhere else since then; too bad.

speed sensor is located in front of engine oil dip stick on my o1 rk,in frt of starter motor,my speedo stoped working today, and the sensor has one 7/16 bolt head easy to get to with open end wrench.remove the bolt and slowly wiggle the sensor and lift up and pull out,my sensor was coverd with metel shavings,i wiped it clean with a clean rag,a magnet is also helpful to remove shavings from sensor but the rag worked fine,put the sensor back and bolt it back in,not to tight just good and snug.it fixed my problem.speedo works great again.

keep the steering wheel in one spot, do not rotate,  check horn by pressing pad in different areas.  If it is intermittent then horn pad is concern.  If it works well trying slowly turning steering wheel while pressing horn pad if there is a point in turning the wheel where horn becomes inop, then the common fault is the horn contact ring inside of the steering column

Take your round glass tray and plastic three wheeled hoop to a local Sears store, open up the microwaves on display, find the one that is still using something similar, deftly trade out yours for theirs. The new wheeled plastic hoop wheels are slightly bigger and the glass tray raised guides are a bit longer, since the switch no problems like I had. Don't get caught. They do not have those parts for sale at the local Sears store.