I am looking into what options we have in securing the traffic between the f5 ltm and webservers? Currently we have encrypted traffic between client and and ltm (client side ssl). I know server side ssl is one way to do this.
Serverside SSL is the best possible option. You can also explore VPN of some kind but it will get too complicated and consume too much resources on the F5 and on the servers.
Normally an F5 device is close to the servers and are passing traffic to the servers within a hop or on L2 over dedicated switch/router and hence, VPN is an overkill, in my opinion.
As others have said, and you've suggested - Serverside SSL - Of course depending on your environment/requirements this could be as simple as a Self Signed certificate or having a full blown CA chain.
Also might be worth fiddling around with Negotiation options, ciphers etc. Depending how in depth you need/want to go.