I went through and replaced a good portion of the cars with modded cars, and now textures aren't loading. Does anyone know why?

You mean the GTA IV world texture aren't loading? This is normal if your using a lot of high-poly cars and your computer can't handle it. Its partly the games fault to but try removing some cars and see if that helps. Or turn your graphic settings down.

I went through and replaced a good portion of the cars with modded cars

There's your problem.

Out of video memory.

Remove some cars and lower your graphic settings

Thanks for the tips.