I have a 03 Doublecab and chatted with my dealership about something like this when I bought it... The Service Manager (who is actually really good) said that some years of Tacos (2000 -2003) were put out with 5000 lbs and some were put out with 6000 lbs. He said that the rating changes were more of a legal and/or marketing item than a vehicle one. Unfortunately I haven't seen any other information that would confirm what he said.
As for towing that kind of weight, I have done it with my truck. I towed a 6x12 (9 foot high) enclosed trailer with a total of just under 5k lbs (trailer and load). The truck doesn't really enjoy that kind of weight. I was able to go everywhere I needed to over the week I had the trailer, but really couldn't hold (or get to) 50+ mph in anything more than a flat area with no wind. If I had to drive with a trailer that heavy every day for work I would choose a different truck.
Now the 2005 model is a totally different beast. It has a tow rating of 7500 lbs and has the engine to back it up. It's bigger, wider, etc. From my experience, I would say that if you are going to tow 4000+ lbs REGULARILY or DAILY, the 2005 model is the only Tacoma to look at. Or look towards the Tundra, Nissan Titan or one of the domestics for bigger size, bigger engine, more towing. (Nothing really tows like a diesel

)
Hope that helps.
Pete