Would driving at 90 mph harm a 4.0 v6 rangers engine?

Its a 2007 ford ranger 4.0 v6 with a 5 speed manual and is in great condition.

No. Its over revving that hurts engines as we get excess vibration and wear etc. As long as we not exceeding its max rev and speed design point then it ought to do fine. Do bear in mind. When we use anything at top end of its ability range it will give rise to potential early failures.
compare for example same 2 cars.one driver fast.braked hard and yet covering same mileage as another but that one is driven carefully with no heavy acceleration or braking. What car do you think will last longer?
so while it technically won't hurt it as such. Its not good idea to do so often or for long periods of time. Plus isn't it hard to find places to run at those speeds?

Yes. Just like starting the car up in the morning puts wear on the engine, so will everything else.

Maybe if you try to do it in 3rd gear, but driving properly in top gear, no worries