  The vortex dynamics of Bi$_2$Sr$_2$Ca$_{1-x}$Gd$_x$Cu$_2$O$_{8+\delta}$
single crystals is investigated by magnetic relaxation and hysteresis
measurements. By substituting $Ca$ with $Gd$, it is found that the interlayer
Josephson coupling is weakened and the anisotropy is increased, which leads to
the change of vortex dynamics from 3D elastic to 2D plastic vortex creep.
Moreover, the second magnetization peak, which can be observed in samples near
the optimal doping, is absent in the strongly underdoped (with 2D vortex)
region.
