You have a sticky caliper. Just have one (preferably both) calipers either rebuilt or replaced and you should be fine.

I had the same problem on my Subaru. I took both front calipers off the car and disassembled them. I cleaned up the pistons and lubed them. I put everything back together and the shimmy went away.

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R