Do I really need to obey the 55mph speed limit on a Uhaul trailer, if I am towing it with a 2001 Sentra?

Yes, the reason is that the tow vehicle can become unstable and you can loose control (its too light of a vehicle to travel at a faster speed) so drive the recommended speed limit and drive safe...

Putting the speed limit issue aside, a Sentra is not really big enough to be good for towing trailers at high speed or on hills. If you try to tow too fast in the desert (New Mexico and Arizona) without stopping, the Sentra will overheat. So the trailer will force you to drive slowly enough to keep the car from overheating. If you take a route that goes through the Rocky Mountains, then the Sentra might not be able to go over 55 mph when pulling the trailer up some of the steeper hills, simply because it might not be powerful enough. So, again, the weight of the trailer will slow down your trip.

Yes more so if you are towing with a older car

that said it's good to follow speed limits anyway you never know when a police man will be there

I moved and took all my stuff in a truck. I wish I would have just packed a suitcase and gone. The furniture and crap I took did not work at the new place and I don't have one piece of it left now. If I ever move state again I will just pack a suitcase and go.

its a safe speed for the trailer i've pulled them faster with no trouble