  Monte Carlo calculations of the irreducible particle-particle interaction on
a two-leg Hubbard ladder doped near half-filling are reported. As the
temperature is lowered, this interaction develops structure in momentum space
similar to the magnetic susceptibility $\chi(q)$ and reflects the development
of strong short-range antiferromagnetic correlations. Using this interaction,
the eigenfunction of the leading singlet pair eigenvalue is found to have
$d_{x^2-y^2}$ like symmetry. The single-particle spectral weight is also shown
to peak near $(\pi,0)$ and $(0,\pi)$ when the ratio of the inter- to
intra-chain hopping $t_{\perp}/t\simeq 1.5$, leading to an increased tendency
for pairing.
