As you probably know, engines don't slip, clutches and automatic
transmissions slip.

If it is a manual shift car, then either the clutch pads are shot or the
tension on the plates is too low or both.  This problem is agrevated by
people who ride the clutch (leave their foot on the clutch when they are
not using it),

If it is an automatic, the first thing to check is the transmission
fluid level.  If that is OK, the it may be bad news.  Loss of fluid is
normally do to a leak.  Most people see the wet spot on the ground where
it is leaking before they notice slippage.

I am not sure, but I think that all XCs are automatics.