THE SOLUTION:
Unser wears #21,
Smith wears #23,
Davis wears #25, and
White wears #34.
Clue 1 = Unser did not wear #25 or #34.
Clue 2 = Either Unser wore #21 or White wore #23, but not both.
Clue 3 = Davis did not wear #23 or #34.
Unser cannot wear #23 (this conflicts with Clue 2) so he wears #21.
By elimination, this gives Smith #23, Davis #25, and White #34.