I know a few on this sight (2000LtdKen and RockyMtnRay for sure) have the Gibson muffler with the JBA headers and have reported good results for towing performance. My main question is which Gibson are you using? Is it one of these:
http://www.gibsonperformance.com/products.asp?pid=18352
I guess my other questions are is it louder, quieter or about the same as stock, with the JBAs? Did you replace just the muffler (and headers of course) or other parts of the system as well, piping for example? I assume that this Gibson muffler needs to be welded into the stock pipes? Without going and looking under my truck I don't recall how the existing muffler "fits in", I seem to think it's part of the tail pipe assembly, all one piece from behind the Y-pipe all the way back.