Mike,
 You are doing the right thing in your method for interpolating form the ruc236 grid. The RUC code itself uses w3fb11 in many places to get (xi,xj) from lat/lon. Don't forget that if you get (60.8,75.9) from w3fb11, that (61,80) is your nearest grid point, i.e., you need to round to the nearest integer values (or interpolate between grid points). I can't comment on what is being done for URET except that it sounds like there are 2 or 3 interpolations involved. If this is an issue, I would recommend comparing values from the direct-ruc236 interpolation as you discuss with the URET interpolated value. The u/v components should be within a few tenths of a m/s, which seems like it should be acceptable.