I just bought my savage X 4.6 big block and I finally got it all tuned... So I was just driveing it down the road and as I was coming back only the front tires were driving. (The back were just rotaining freely) what do I do??? Please help!! Thanks !

Oh... The rear diff might have gone.
You probably have to replace the crown gear.

Picture of it.

Removing it.

Check the set screws in the read part of the drive train that hold the drive cups, they may have gotten loose allowing things to just slip.

Also while you are at it, check inside the diff and look at those gears. My Savage used to blow those out a lot, but that was when they were still using the 2 gear bevel type. Ever since I upgraded to 4 gear bevel types I haven't had a problem for years. Maybe your truck just got a bad set of gears with it if it is still newish. These things happen, not a lot of money to replace those.

Any chance the rear drive shaft came loose?

First we have to learn about the mechanism and concepts of four wheel drive system in order to find out the root cause why rear wheels are not driven.

The 4WD systems direct power to all four wheels, some of the time or all of the time. Broadly speaking, "four-wheel drive" refers to systems with a two-speed transfer case, designed primarily for low-speed, off-road driving. Mostly, only the rear wheels are driven in normal operation, with four-wheel drive selected only for extreme conditions. So the rear wheels are burdened most of the time, hence they are likely to get out of order more than the front wheels.

Sure you need to check the rear drive shaft.

One more thing. Since you are going out for a drive on your new vehicle so I suggest you to scale your vehicle on a good quality weighing software

I have no idea which vehicle you are going on about, but we are talking about an RC HPI Savage truck. The Savage doesn't have a center differential, so its an All Wheel Drive system. The power of the engine is transmitted equally to the front and rear, but its up to the differential on the front and rear to split the load to the wheels.

So Miller18, if the rear wheels are not receiving any power while the front is getting power, that tells me a few things.

Since there is no center differential and the front is receiving power, that means that there is nothing wrong inside the center transmission case. Next in line is the driveshaft cups at both ends of the driveshaft. If the setscrews are still there in both cups, then it is a problem inside the rear diff. Most likely, the bevel gears inside the diff are ruined or the gears just outside the diff. Another problem may be that the drivecups coming from the diff to the wheels are missing their setscrews as well. With an open style diff that is used in the Savage, only one setscrew needs to be missing in this area for the rear end to show no power. The diff will send the power to the wheel of least resistance, and since there is virtually no resistance to the wheel with the missing setscrew in the drive cup, then it will send all the power to that area.

So check those areas and let us know what you find.

easy check all of your gears and shafts dont forget to get your hands dirty