How can I find the equivalent resistance, Rxy, of the two NMOS transistors in Figure 5.25 in Razavi's "Design of Analog CMOS Integrated Circuits"?
To find \$R_{xy}\$, we redraw the portion of Figure 5.25 of interest below, substituting small signal models for M1 and M2 (with \$\gamma=0\$,     \$g_{m1}=g_{m2}=g_m,r_{o1}=r_{o2}=r_{o}\$), and apply a test voltage \$v_{test}\$ across the terminals:



From the above network we can use KVL to write, 

\$v_{test}=(i_{test}+g_mv_p)r_o + (i_{test}-g_mv_p)r_o\$

\$v_{test}=2i_{test}r_o + (g_mv_p-g_mv_p)r_o\$

\$R_{xy}=\frac{v_{test}}{i_{test}}=2r_o=2r_{o1,2}\$.