Short answer is NO. you are not... if you have 3 on the tree, it's a near guarantee that your truck is old enough not to.
anything pre Sept 1 73, no bumper.
LIGHT trucks pre 98 model, also appear to be excluded.
WI
305.18 appears to be the section that would cover this, according to the following.
http://www.dot.wisconsin.gov/drivers...violations.pdf
Trans 305.18(1)(a)
(a) Every motor vehicle manufactured after September 1, 1972, and required by federal law to be equipped with bumpers or crash resistance protection, or both, shall be equipped with bumpers which shall be maintained in good condition and in conformity with this section. Replacement equipment shall be the same type as originally required or of equivalent size, strength and surface area. The bumpers may be mounted externally on the vehicle or installed within the front and rear extremes of the vehicle.
Trans 305.18(1)(c)
(c) Light trucks manufactured on or after January 1, 1997 are required to have rear end protection in accordance with s. Trans 305.56. Light trucks operated under s. 341.47 (1), Stats., are exempt from the requirements of this paragraph.
341.47 When vehicles of dealers, distributors, manufacturers and transporters exempt from general registration requirements.
of all the technically illegal things done frequently and blatantly, like air rides that violate scrub line laws, violation of bumper height laws, headlight height laws, modifications to emissions equipment, violations of lighting codes, etc... a rear bumper (or even a front one) on an old truck is pretty low on the list.
You;d win, but at what cost?
remember Though, License plate illumination is STILL required for the rear plate, even sans bumper.