I'm going to lift my 98 tacoma soon and want to put 33 inch tires on the truck. The tires I'm looking at are 33x12.50x15. Will these tires fit on the stock rims that came with the truck?

 

12.5's don't fit on stock 7" wide wheels very well. They'll be ballooned.
Something closer to 11" would be better.
A 285/75/16 would be good, but you have 15's. Not sure what comes in a similar size for a 15.

If you put a 12" wide tire on a stock rim, you're going to rub not only the UCA but the frame rails and the pinch weld.
Some 1" spacers would get you away from the UCA's and the frame, but you'll still have to pound the pinch weld.

 

What about a 33x10.50r15?

 

Those will fit better than a 12.
They won't be ballooned on the rim. You will still get some frame rub on full lock steering but that's not a huge deal.
You will have to trim some plastic at the back of the front fender wells as well as pound down the pinch weld some.

I had 33x10.5's on my truck for a couple years. Great size.

Check out my build thread (link in my signature) and you'll see plenty of pics of my truck with those on.