91 Nissan Power Steering Fluid Leak

If the cap is leaking around its seal there is something impeding the fluid and it is creating high pressure from the blockage. If the fluid level has ever been overfilled, that could cassue a seal to blow internally and block a passage.
The system will need to be flushed and the blockage isolated and cleared or the component replaced. Don't hesitate to get it fixed as the lack of fluid to areas beyond the backup need fluid for lubrication and will fail or get worn just like engine components.

I have never seen power steering fluid steam. It is oil based and if there is a whining noise, the power steering is blocked or the pump is seized.
The power steering rack and pump probably will both need replacement and the hoses flushed and looked at. With the issue you have, I would get new hoses. This is not going to be cheap, but things like this do happen and especially steering issues with SUVs and 4X4 Trucks. Sometimes big diesel pickups have an integrated hydraulic system for the steering and the brakes.
Get to a repair shop and get an opinion or two. Depending on how long you are keeping the truck, you will have to weigh out how far you carry the repair, i.E. A band aid or the whole 90 yards.
I have never seen or heard of a power steering system blowing or leaking around the cap and never the steam. That is something else.
How does the steering feel?
Does it whine more when you steer it at a dead stop?
Is the fluid brown or smell burnt?
Let me know how things turn out and I. Or someone on our team, will be here to help you.

Streaming. Okay, I was really concerned that your steering was melting.
There are some possibilities;
There is a pressure switch that changes the amount of pressure allowed to assist the steering based on vehicle speed. Otherwise known as speed sensitive steering. For many japanese vehicles of the early 90's, it is a bypass that comes off the power steering, to a gear drive where the speedometer cable goes into the transmission where is is pressurized by means of the speed that the cable is spinning. It then travels past a pressure sensor switch that, I forget exactly if it bypasses or if there is a valve, etc, to make the steering less responsive at higher speeds so you don't change lanes into the oncoming traffic. There is also a setup that has basically two bypasses with different size restrictor and based on speed, one of the two is used for low speed or high speed.
So, there very well could be an electrical component that is not intermittently restricting the pressure at high speeds and over pressurizing the system.
This is actually pretty likely, the question now is what components is causing it.
Since it is intermittent, I would try replacing the power steering pressure switch, or at least test it with a multi meter. A haynes manual should have the spec to test it.
If it is over pressurizing at highway speed, the high pressure hose could blow and that is not the best fluid to be on while changing lanes at 65 MPH.
I think someone who has experience with Nissans, meaning try to find a repair shop that advertises itself as a Nissan or at least a foreign car specialist, since it is an unusual problem.
Let me know how it turns out.

That is great news. I am glad that is all it took to fix it.
I appreciate the reply to let me know the outcome. It is always a good feeling to know things turned out okay.

Take care

Dr. C