99 Sierra *VERY HOT* exhaust

I've got a 99 GMC Sierra 3500, 5.7 Vortec 4x4 that I just replaced the muffler on. I've notice before I replaced the muffler that the exhaust seems to get extremely hot. The engine doesn't overheat - in fact it's pretty cool. It runs great and there isn't any smoke or anything like that. I'm just wondering what I might look for that would let me know if it's getting "too hot". The new muffler has purple on the front end of it and if my memory serves me correctly, I think it means it's too hot. I don't see any restrictions or have any SES light...no codes.

Since no one has replied as of yet, I've gone and done a little diagnostic (and research here). I've tested the fuel pressure, and it looks normal according to everyone's postings for fuel pressures. The next thing I looked at is the EGR Valve. I have an extra one from another truck (we have 15 here) so I thought that I would just swap them and see if that is the issue. When I pulled the valve, there was no gasket on it. Stunned (well, not really since I fired the old mechanic for things like this), I went to the parts house for a new gasket. $3 later, I'm in front of this pc letting you know where I am in the diagnosis.

I'll also offer up that I got 4 codes that came up since the original post. P0131, P0137, P0151, and P0157. I read in another post that the cats may be plugged with these codes - and the pipes coming from the engine to the cats are a different color then the typical "light brown" I'm used to seeing. I've tried to pull the O2 sensors, but they're in there pretty good. I haven't gotten them out in fear of breaking them. Most things on this truck are pretty well rusted in place due to it being a snow plow truck that at one time carried a tailgate salter.

So, I'm going to replace the EGR gasket, but does anyone have a better idea of what to look at? I'm very appreciative to anyone's ideas (other than driving the truck into a tree).