I have a 2006 2500 Diesel 4x4 Quad Cab, long bed. Want to know how much trailer I can pull before I have to consider a bigger truck.

Yeah, I figured it out. I used two different sites (one gave me 2008 info and one gave me 2006)

http://www-5.dodge.com/vehsuite/TowingGuide.jsp

http://www-5.dodge.com/towing5/D/vehicle_to_weight.jsp

On the 3500, the pictures were always duallies, but I am PRETTY sure you can get 3500 single wheel???

For the 2008 Dodge 6.7 Cummins SLT Quad Cab 8' bed 4x4, I found the following:

2500 6sp manual tran - can tow 12,700#

3500 6sp manual tran - 13,650#

3500 6sp auto 68RFE tran - 16,700#

For 2006 5.9L HO Diesel, 4x4 quad cab, 6sp manual tran, SLT, 8' bed:

2500 - 12,750#

3500 - 15,300#

All Single Rear Wheel Superduties with the 6.0L diesel have a 3.73 rear axle.

So for the 2006 F250 Superduty 6.0L SWB, Extended cab 4x4, you can tow 15,700 lbs with a Gooseneck or Fifth Wheel.

For the same truck pulling a bumper pull the limit is 12500

Your biggest problem will not be the total trailer weight, but rather the Pin weight. A 15,700lbs trailer will probably have more pin weight than your F250 will carry on it's rear axle.

I have a 2006 F350 CC SRW LWB 6.0L and my horse trailer is around 16,000lbs when fully loaded. When I go across the scales at the POE my truck front axle is under the max, my rear axle is real to close to it's max rating and the rest of the weight is on the trailer axles (which are under their 14,000 ratings) My total GCVW is just 200-300 lbs over the rated max for the truck. So if my water tank or Fuel tanks are empty, I'm under, If I'm topped off, I'm slightly over.

A F250 EC SWB 4x4 with the Diesel/auto has a cargo load of 2800lbs in the bed.

My F350 CC LWB 4x4 with the diesel.auto has a cargo load of 4000lbs in the bed.

So chances are you won't be able to tow any where close to the Max tow weight because of your cargo weight limitation. I suspect a trailer more in the 11,200lb range, but that will depend on how much of the trailer weight is on the pin.