||We use ac susceptibility measurements to explore the dynamic response of vortices in YBCO single crystals pinned by aligned columnar defects at low de fields. Using a combination of techniques for the analysis of the data, we investigate the vortex motion as a function of the amplitude and frequency of the applied ac field h(ac), identifying the influence of both intra- and inter-valley motion. We build up a dynamic diagram in the h(ac)-T plane indicating the crossover lines among several regimes in the solid phase. At low h(ac), a linear response with very low dissipation arises from the oscillation of pinned vortices inside the tracks (Campbell regime). At high h(ac) a critical state develops. The large influence of thermal fluctuations in this limit is indicated by the frequency dependence, which allows us to obtain the current density dependence of the activation energy and the glassy exponent. We also discuss the origin of the large nonlinear transition regime observed at intermediate amplitudes. [S0163-1829(99)02613-2].