What is Jerry Cruncher's secret nighttime activity, and what important theme does this activity underscore in A Tale of Two Cities?

Jerry Cruncher's euphemistically-termed occupation as
"resurrection man" proves pivotal to the plot of A Tale of Two Cities
while he is still in England as he observes the funeral of Roger Cly.  For,
later, after the funeral, Jerry decides to rob the grave and discovers that the coffin
is filled with dirt and rocks.

Related to the theme of
resurrection, in Chapter 8, "A Hand at Cards," of Book the Third of
the novel, in a fantastic coincidence, Jerry and Miss Pross are shopping for groceries
and run into her brother Solomon, who is, in fact John Basard, the spy and the "Sheep of
the prison."  Then, another coincidence occurs as Sydney Carton and Mr. Lorry appear on
the scene.  Carton looks at Basard/Solomon and recognizes in Basard--"I know the
face"--the man who testified against Charles Darnay at the treason trial for the French
aristocrat.  But, this Roger Cly, whom Basard claims the witness was, Carton observes,
spoke "good French.  Yet like a foreigner."  He then
says,

"Cly! 
Disguised, but the same man.  We had that man before us at the Old
Bailey."

Now Carton has
"cards" to play against Basard, the jailor at the prison.  But, Basard contends that
Cly, who was his partner, has been buried in London.  It is here that Jerry interjects
his knowledge that Cly was never truly buried, revealing that the coffin was empty. 
This gives Carton the edge that he needs, the cards to play, to control John Basard. 
Thus, he is able to have Basard, who fears his exposure to the revolutionaries, let him
into Darnay's cell so that he can switch places, resurrecting Darnay from prison as his
father-in-law was also resurrected, and thereby providing Carton a spiritual
resurrection as he becomes the sacrificial victim for Charles
Darnay.