It's not difficult to replace the seal at all. If you don't already have one, a seal puller will make life a little easier (they are just a few bucks at auto parts store). One thing to check is to make sure that the circular spring is on the right side (the spring squeezes the rubber part of the seal around the shaft). I'd check to make sure that the nut that holds the yoke is still torqued to spec as well before I jumped to replacing the seal (especially since you recently did the SYE install)
Other than that, the seal could have been damaged when installed and you may just have to replace it and be extra careful not to damage it.