First of all, you should consider the relationship between the WGS84 lat/lon/alt and the ECEF frame. Altitude is perpendicular to the reference ellipsoid, so it is effectively parallel to "down". I would assume that if you want to know the slope encounrered by a vehicle, that implies the rate of change of "down" over the horizontal displacement. So when you tranform your data to ECEF, you lose the "down" reference.
Rather, you should be working in a local level frame, such as North/East/Down (NED).
You can transform ECEF to NED using;
In this (NED) frame, X points to the north, Y points east, and Z points down (i.e positive z is down) . Now if you calculate delta Z over the X-Y distance (in the NED frame), it should come out as the parameter you are seeking; the change in height over the change in horizontal distance. This would be the tangent of the local slope angle.