My 95 PSD uses 3.73's and I averaged about 19.1mpg overall until I parked it. Towing, it dropped to about 15mpg w/ a tall trailer, but 16-17 with my car in tow. Pulling a loaded trailer was never a problem for me, seeing that mileage even at 70mph. Low end isn't a problem either, since diesels are torque monsters.
3.55's aren't all that different, but either would be better on fuel then 4.10's, which are fairly steep for road use anyway. Noting the economy differences you have, 60mph might just be a fine answer! In that part of the country, you're not far below speed limits anyway. 3.55's should allow better mileage and no worries towing as you do.