Trying to get the time difference between
"Service #1" Service Completed on 04/15/15 01:55:12
and
"Service # 2" Service Completed on 04/15/15 05:25:31
and then the difference between
"Service # 2" Service Completed on 04/15/15 05:25:31
and
"Service # 3" Service Completed on 04/16/15 10:16:11
and then the difference between
"Service # 3 " Service Completed on 04/16/15 10:16:11
and
"Service #4" Service Completed on 04/16/15 14:57:12 

That was it! Thank you! I really appreciate the help! Seems so simple now :-)

Since I only needed it to count the time between service for WO with more then 1 service.
I created a running total to count the number of services on each WO. I then used your suggestion for Previous() Function. I also divided by 60 to get my hours. Then suppressed the field if Total Service count = 1

if {#WO Service Count} > 1 then
datediff('n',{Service.CompleteOn},previous({Service.CompleteOn})) / 60.00
else 0