I have an 08 F250SD Diesel 6.4. I plow and tow a 5th wheel. Not at the same time.
I am new to towing the 5th wheel and have a problem. The fifth wheel is 11,000 pounds. When I hook up and start out, the truck revs to 3500 or 4000rpm and will not shift. After a few moments, the wrench light appears on the dash and the truck runs perfectly and shifts perfectly. The warning light is listed as to report a transmission problem. After I unhook the 5th wheel and start up the truck the light is out and stays out. It only comes on when towing and will go out if I stop the engine, but when I restart I go through the whole revving until the light comes again. What is going on?

 

The wrench light is simply an indicator that there is a problem and a code is set. The only way to find out what the problem is will be to have the codes read.

If you're still under warranty I would have the dealer look at it. They are most likely to have the proper equipment to read all of the codes.