Calculating Resistance
R_o2 is the impedance looking into the drain node from the perspective of \$V_t\$. That drain has two other connections: -


A current source that has infinite impedance (irrespective of what is connected to the source) i.e. it is the actual drain connection and,
Resistor \$R_D\$


Hence the signal impedance R_o2 is \$R_D\$.