I have the same problem running within those speeds, at first I though of worn out steering rod dust boots but when I checked they were okay. Tried the rims but they are good too,until I changed all tires and the shudder disappeared. The culprit tires even though they appear new but may have bumps due to manufacturing defects or the way it was treated like a hitting a curb hard which loosen the nylon treads inside. The tire was under warranty but I chose changing all fours(got a discount on the warrantied tire).
