I have a 99 Blazer LT 4door 4x4. I want to put 31x10.5x15 tires on my factory rims. My questions are, 1) do I need to lift it to fit this tires? 2) if so, how high do I need to lift it? 3) will that size tire fit on my factory 15'' rims?

 

1) Yes if you do not want the tires to rub on the frame and fenders.
2) A minimum of 2-4 inches. Check out this thread.
3) Yes they will fit fine on factory rims.

 

jsschlimm.... welcome aboard....
The funny thing about blazers is that it seems that they look alike
But are personally different... when it come to lift...
Some guys have no problem with larger tires and some guys rub
All day long...personally Im a fan of body lifts...a lot of folk dont like
Them, but it give lift and you keep the same factory ride..
Just keep digging here on the site.. and all your questions
Already have answers...its a matter of finding the answer