I was wondering how much of a lift can be done before you must lower the front differential...

 

That depends on a few things... The amount of body lift you have, the size tires you have and possibly the extremity of the off roading you will be doing.

 

1.5" is what is generally regarded as the safe limit, regardless of BL or use, before you will start to see accelerated wear on the ball joints and CV shaft. Pushing it to 2" works for some, but others have chronic problems with the aforementioned items.