How big is your church? If you have 2 full time pastors, I am guessing you run the range of 100-150? I am not sure that is a large enough church to justify a full time worship leader, but...you may consider taking on other roles within the church that are currently under staffed to justify an additional staff person. I know in some cases the youth pastor is also the worship leader or the small groups/family pastor is also the worship leader. I am full time in the business world...the way we justify a new person has to do with the return on investment. At some level, the decisions around staffing may need to be viewed from a business sense as well as biblical perspective.
Praying for your back situation - I can relate.
11 years of leading, 3 services a week, over 200 funerals, 75 weddings and ive still never taken a cent.
Though I have wedding crashed a few ;)
Our church toyed with the idea of have a paid position, but in the end they found they couldn't afford it. Oh well. The advantage of being volunteer is that you're less likely to be fired, and no one can argue with you if you decide to resign. I led off and on for eight years, and still fill in once a month. What I did do now and then was to refer to myself as a "interim worship leader" until the church could hire a professional. That kept everyone in the proper perspective. I felt it was my role to hand over a healthy, well organized, musically tight ministry, focused on the Lord first for the day when I could hand over the job to the one who was to take over. The one thing I also did say was "that if they were going to hire someone to lead, he/she better be better than me!"
But if it's too much, confide in your pastor. If you get grief, quit now. Guilt is unhealthy and breeds bitterness.