I bought my 06 FX4 as a left over a couple months ago. One of the things it had was a 3.55:1 rear end which I wasn't too happy about, but since I was told it was fairly easy (and cheap) to swap it out, I figured no big deal. So, I was going to upgrade to the 3.73:1 rear end, but now reading on the forums a bit, I see people who lifted ended up with the 4.88:1 and it appears even the 4.10:1 is a valid ratio. (Keep in mind, I'm new to this stuff, so my questions may be a little noobish)

So, assuming I plan to lift my truck (eventually; using a rize 6" kit most likely), would it be safe to go with the 4.88:1 now, assuming that's months away, or would I be better choosing the 3.73 or 4.10? And do I need to swap both a front and rear diff? How do the 5.4L modulars handle gear changes? I would assume I could use any of the 8.8" diff gears offered by Ford Racing (for front and rear)?

 

As far as doing front and rear, you have to do both or you can not use 4wd. Also, if you are not going to go to larger tires before you do the lift, there is no reason to change gears now, then again later. If you go now with 4.88 and keep the OE tires, gas milage will suffer and you will be over working the engine with a higher rpm.

 

what problem will happen if you put it in 4x4 with a different gear ratio in each end?

 

Transfer case will go KABOOOM!

 

If that is indeed a serious question - I'll give you the benefit of the doubt - any two systems connected mechanically but operating at different speeds will stress until the weakest component fails, given enough torque & traction.

In this case, given enough available traction, it'll be either the diffs, axles, joints, x-fer case, etc.

Both ends must be geared identically. Period.

 

Thanks for the info, I will keep the 3.55 for now until I do a lift, or move to the 3.73 if I decide to not do it at all.