is it only the front tires that seem to have power going to it? If so, try tightening up your rear diff. If both front and rear have power going to them but the front is spinning more, then try loosening up the front a tad.

Check the drivetrain for anything loose or inconsistencies with diffs. It might possibly be a stripped spur brace in in the rear or you could be missing a pin.

also check the dogbones or universals in the rear. You might be missing a pin there as well.