Exactly what versions of IOS are running on these devices? Please post their running configs. It would also be helpful to get the result of an snmpget on rttMonApplResponder.0 (.1.3.6.1.4.1.9.9.42.1.1.13.0).
You do not need adventerprise for IP SLA, but you do need something higher that IP BASE.
Hit Post too soon. If all you want to do is ping a router, you do not need the responder enabled. Simply create an IP SLA icmp-echo operation on router router to ping the other. That target device doesn't even need to be a Cisco IOS device. It could be any IP-enabled node.