Toyota Repair: 99 Toyota Tacoma, muffler system, toyota tacoma

Question
Check Engine Light - Repaired 2 times within 5 months. O2 sensor has been replaced and code has been reset.  Muffler system is just beginning to show signs of aging.  Some rust(dusted away)and bolts to tighten system has been replaced.  Is it time to replace Muffler? Mechanic has been putting it off, he doesn't think it is 100% ready to be replaced. Claims that the job is a lengthy job.  Trying to put it off as long as possible.  Mechanic is a Toyota registared repair center.  

Answer
Is the check engine light on again and if so I need to know what the trouble codes are.
Mufflers don't need to be replaced unless they are leaking or have some other problem with exessive noise, so I don't know if the muffler needs to be replaced, is there a specific problem with the truck?
If so let me know and I can help.