You should payed attention from which IP your are sending the ICMP echo request.
The source IP address of your testing packets should be in the participating networks of the IKE phase2.
For exemple :
On your 2811 use the following command
ping destination_ip_add source 2811_lan_ip_add
or
ping destination_ip_add source lan_interface_name
on your Juniper (if screenOS) use :
ping destination_ip_add from lan_interface_name