I had an older (2000 or 2001) E350 with 6.8l that towed my TT one summer. I am not sure which gears it had (3.55 or 3.73) but I do remember when I was cruising at highway speeds (65 mph) and in OD I was good on level ground but if I tried to lightly accelerate (or incline) the engine lugged until it dropped out of OD. So I either cruised faster or forced it out of OD by giving it more gas.
My Excursion 6.8l with 4.30 had a much more useful power band. I could cruise at that speed with cruise control on up slight grades with no issues.