What is the difference or relation between Rd and Rf in a diode?
You have an equation for the DC dynamic resistance- the Shockley equation, minus the ideality factor (Edit: it has now been added in). And it's valid- at a few hundred Hz, certainly.  

However to, quote from this app note, "The DC dynamic resistance point is not, however, valid in PIN diodes at frequencies above which the period is shorter than the transit time of the I (intrinsic) region. The frequency at which this occurs, fT , is called transit time frequency". 

Above fT, the resistance is still inversely proportional to applied current, but is significantly lower. The proportionality factor depends on the physical parameters (Intrinsic region width and electron and hole mobilities and carrier lifetime). 

\$R_S = \frac{ W^2}{(\mu_n + \mu_p) \tau I_F}\$ 

Where W is the intrinsic region width, \$\mu_n\$ and \$\mu_p\$ are electron and hole mobilities and \$\tau\$ is the carrier recombination time. 

We can predict fT as follows: 

\$f_T = \frac{1300}{W^2}\$ with W in microns and fT in MHz, so in the case of the Infineon diodes W = 4.5\$\mu m\$ so fT = 64MHz.