Not to sure what that means but have read it abit on this forum. Does it have anything to do with space around the rear tires?
I'm asking because I'm sure when I put 37's on (currently have a 3.5" w/ 35's) I'm going to have to make space between the rear tires and the front portion of the fender surrounding them.

 

Back space refers to the distance from the mounting portion of the rim (where the lugs go through) to the inner bead seat of the rim. This has to do with wheel clearance to the suspension components not the wheel well or fenders. Hope this helps.

 

Here is my shot at this.
Backspacing is the distance from the back lip of the rim to the flat surface that you mount your wheel to the axle hub. So - a 4.5 inch back spacing would be 4.5 inches from the flat mounting surface to the level of the lip on the rear of the rim.
This spaces your tire and rim out from the axle mounting surface giving you a bit more clearance on the inside frame and components when turning the wheels/tires.
When lifting mine and buying rims and 35's, the suggested backspace was 4.5 inches. I purchased rims with that spacing built in but many here just add a spacer to the mounting surface. Usually about 1.5 inch.
It will make a difference in body and bumper clearances only because it has pushed your tire out a bit and when turning it may be close to fender or airdam in the front.
Stay safe
Mandor6863

 

I'm thinking that if you are going to move up to 37's and you don't know what backspacing is, you have a lot of research to do. You might want to read through the FAQ sticky at the top of the Modified section.
This image is from the Wheels / Tires section


There is a lot that goes into putting 37's on if you don't want to tear up a bunch of components.

 

Ok that makes sense. I've had 5 spider spacers (1.5" ea) I was originally going to put on when I did the 3.5 lift but saw I didn't need them except for when I had my stock bumper- used one spacer to push the spare away from the bumper.
I have the AEV lift with Billstein shocks, heard that system was pretty friendly when making adjustments. I was just gonna throw the 37's on but if its true what they say, I'll throw an extra inch on the lift and either way use the rest of the spacers.

 

there will be other needed (or maybe just recommended) upgrades in order to run 37's besides just backspacing and fender room. stock axles may not fair too well with 37" tires.

 

Wheel backspace is simply how far in/out of the wheel well the rims/tires sit. Stock BS is 6.25". Adding 1.5" spacers to stock rims pushes them out, making the effective BS 4.75". (same as an aftermarket rim with a 4.75" bs).

But what you are referring to is trimming the pinch seam in front of the rear tires (and possibly trimming the rails, if you have them?), and/or moving the axle backward in the wheel well using new upper and lower control arms.

 

A few things I'm kicking myself over is, for one, not going for the 6.4 rather than the 5.7 hemi. I didn't want to have to upgrade to a D60 ax. But seeing as one of these days I'm going to...