Finding Peaks in an Autocorrelation Function
Removing the DC offset from your signal will get rid of the triangular "trend" seen here. Another way to detrend data (which is not specific to autocorrelation functions) is to subtract from your function a median-filtered version of itself (the median-filtered version corresponding to the trend).

You can then detect peak by detecting local maxima - if $X(n) = \max_{k \in [k-W, k+W]} X(k)$, then $n$ is a peak. $W$ is a scale factor which indicates how narrow and close to each other you allow your peaks to be.