So I just went from 31x10.5 to 33.12.5 with new wheels on my 2002 2wd ranger and they look awesome! With a little trimming on the valance everything fit no problem. Before the switch I was getting a little bit of steering wheel shake while braking but now it is exponentially worse, plus a little bit while cruising at 45.

Any ideas on what the problem could be?

 

Shake while braking is pretty much 100% warped rotor(s)
Large tires = larger weight = more shake.

Put wheels straight, remove front tire.
Rotate rotor while watching caliper, you can usually see the warp.

At speed it causes the caliper to vibrate, i.e. shake at 45MPH

Warped rotor can come from overused braking, pulling trailer alot or using brakes alot on long downhill grade, metal gets hot and warps.

Or a dragging(bad) caliper can cause rotor to overheat and warp.
So if you have to replace a rotor.....caliper should be replaced as well.

If there is a shake and it stops when braking then wheel bearing or tie rod end

 

I read up about steering wheel shakes here Car Shakes and Steering Wheel Vibrates Maybe you have to get your tires balanced? Hope it gets fixed.