I have a 2000 250SD 4x4, extended cab with the 5.4 and it tows my 32 Fever with absolutely no problem. Just a bit slower getting up to speed, but once there, I can cruise right along at 80. Mine seems to have a dead spot at 65 while towing where it is harder to maintain speed without alot of throttle jocking, so I just tow in the 70's. It actually tows easier at the higher speeds. I assume because it is running higher up in the torque band. I'm probably in the same tow weight area as you.
Can't comment on the 250 vs 150.