I've some experience with the 811mj - differences being pin distance and some minor features - for ethernet connection.
I remember I had difficulties with pinging also.
Just in case you did not have a chance to look at the DHCP libray,
you may well have a look at the thread
(Arduino Forum - Wiz811MJ with Arduino for ethernet connection) including
DHCP library(credit: Jordan Terrel, http://blog.jordanterrell.com/post/Arduino-DHCP-Library-Version-04.aspx
Spinlock's suggestions in the thread regarding ip address designation method was also a great help for the situation.
Hope this will help the situation.