The TM says the M1009 should not be pulling an empty M101 trailer. Much less with a S-250 loaded on it. If you haven't taken your pintle hitch mount apart and inspected/replaced all the fasteners in there. I would strongly tell you don't even try. Even after making sure it will hold. You are going to have a hard time going anywhere above about 52 mph.
The 6.2 is great at low rpm torque. That is what makes the M1009 by itself such a fun vehicle to drive around. Adding a 1200 pound trailer makes it much less snappy. If you drive around with an empty M101 without its top on, you will find you have to push a lot more to get above 60 mph. Put the top on the trailer and if you have a head wind. You might be on the floor, blowing black smoke out the exhaust and maxed out at around 52-53 mph. A square truck with 3.08 axle gears pulling the barn door into the wind needs power the 6.2 doesn't have. I have been in the just described situation a few times with my truck.
Add in the increased size and weight of a S-250 and you might find your new max speed is around 43 mph. Hopefully, you will be able to get it to shift to 3rd.
I would suggest 3.73 axles if you want to pull that much behind the M1009. A better hitch and for sure good brakes on the trailer. A bigger engine oil cooler and a bigger transmission cooler as well.
