Yes.  A sketch:

Taking products with the free G-space EG commutes with the pullback diagram (because product is also a limit) and so you can assume they're free, and one of the maps is a fibration.  

Having done this, there is a natural long exact sequence of homotopy groups
<pre>-> pi_* (U) -> pi_*(U_{hG}) -> pi_*(BG) -> ...</pre>
and applying this to the pullback diagram you can deduce (from the 5-lemma) that the natural map from the orbit of pullbacks to the pullback of the orbits is a weak equivalence.