Woody -
I would think that your setup would do better than that. Have you had your van checked over by a mechanic? Something silly like a clogged air filter or dirty fuel injectors could cause it to seem underpowered.
Sometime in the last year, a member had a similar issue with (IIRC) a small diesel Sprinter. In the end, I think the problem was a simple valve somewhere in the air induction system. New valve yielded much improved performance. Read here
http://www.trailmanorowners.com/foru...ad.php?t=10408
As for changing the rear end gearing, it shouldn't be hard for a reputable service shop to do. Years ago, when this forum was new, RockyMtnRay changed the gearing on his Tundra. It wasn't cheap - of course he had 4WD so he had to do it twice - but he reported that it made a substantial difference when he was towing in the Rockies.
Bill