Auburn posi unit

I bought a used 1970 8.2/10 bolt Auburn posi unit and had it installed on my 69 firebird. This units does not use clutchs. A new install kit was used with my existing ring and pinion (3.23). When I drive it, barely moving sometimes, it makes a clunking sound even with the clutch in. I notice it mainly when I am pulling it in and out of my garage going straight. The u-joints seem to look OK. Could it be the type of oil the mechanic used. I believe he used Synthetic with anti slip additive. Does anyone have any thoughts?

Thanks