tire rim weight rating

Markings are on the outside of the inner portion of the rim.
If spare is the exact rim, you could check it.
Otherwise you will need to crawl under rig. Use brake clean and rag to clean it to make it easier to read. Letters/numbers are like 3/16 inch in size

Makes it easier to have tire off, it can be achieved by crawling underneath with flashlight or cell phone. Maybe a selfie stick for your phone would make it easier, I found most rims are specked 10< psi than max psi of the tire that was originally mounted on the rim.

I looked up and found the factory window sticker for my truck and the wheel ratings were on it...

Not all rims have weight rating on them as was done some years ago. It can be tricky to ascertain this data. My factory rims had no such numbers, inside or out.

When I was going through all the arithmetic on my last set-up, the one thing on which I was fairly certain: The wheels at a minimum were able to handle at least the weight of the stock tires from the factory that were wrapped around the wheels. Sounds simplistic, but I imagine the lawyers for the truck manufacturers would not allow for a higher tire rating to be installed on a wheel that would not be able to at least handle that.

This does not mean that in the real world that the wheels are not capable of more loading, its just that the truck maker will not stipulate it.

FWIW- when I calc'd my set-up, I was over the wheels (per the above assumptions) by 75# per wheel but well under axle and tire ratings since I upgraded the tires.
I was comfortable with that weight on any known wheel rating since these were steel wheels.

SO the tire needs to be off?

Rating is stamped on the inside of rims.
Wheel identification