can i put a 4th generation rear end into a 3rd generation easily

 

It will bolt right in, no problems.... except the 4th Gen rear axle assembly (if you are talking about the complete assembly) is 3" wider than a 3rd Gen rear axle. You would need to switch to 4th Gen offset wheels.

 

i dont get what you mean by offset does it mean i just need 4th generation rims

 

Yup that's what it means. If you stick with your current 3rd generation wheels they will stick out 3 inches on both sides. Basically they probably won't fit anymore and will rub all the time. Switch to either 4th gen wheels, or aftermarket wheels that have the same offset 4th gen wheels have. Then you'll be just fine.