I wanted to wait to reply until I was done towing this weekend..
My truck is very similar to yours. '96, 4x4, SC, 4.0L, 3.27 gearing.
This weekend I towed an 18x7x7 tandem axle cargo Haulmark trailer almost 300 miles with no issues what so ever. I run 60mph @ 2500rpm. The only issue I had was on starting from a dead stop especially on hills. Without a granny, it's tough. I was able to keep it at between 60 and 65. Most of the trip it pulled so well that I didn't have to think about it. It stands ~8.5' tall at the nose. That was empty, so it's right at 2500#.
I then picked up a 14x7x6.5 V-nose single axle Haulmark and loaded 2500# in it (4000# gross). Towed it 250 miles, unloaded it and ran it back another 55 miles empty. I ran up the Mississippi side of Missouri half way up Iowa through the rolling bluffs. NEVER downshifted from 4th gear and even out accelerated one truck and trailer on a hill. On one hill I got knocked down to 50, but only on one and that was loaded. On the empty trailer, I could maintain 70-72 in 5th just to test it for about 20 miles. It couldn't hold on some hills and rather than lug it, I just dropped to 65 in 4th.
The only thing that may make a difference in what your truck and mine is capable of is that I have a topper on. I can't believe that 2 sleds would cause that much of a drag. That 18' cargo trailer stood well above the topper on the truck.
I now have to go back and pick up the 18' loaded. Total weight is 8700#. BUT it's 1/2" above the nubbers, so it's not overloaded... right?
Added .. one more trip:
Just finished moving my daughter's family from Iowa to Lexington KY last weekend. We put the HHG in the Penske truck and I towed the '95 Cougar on a 20' flatbed trailer. Trailer and car were 6300#. I ran in 4th @ 60 - 65 the whole 525 miles. Though the trailer has brakes, my brother hadn't hooked them up so left plenty of room in front especially in traffic. The Ranger actually handled the weight (gross 10,580) without an issue. Towed the 2200# trailer back the 525 miles empty. Both directions the Ranger averaged 14.5mpg. Going out it was @ 60 in 4th, coming back it was 70 in 5th (mostly).