With the truck off the ground, hold one wheel at a time and give it a bit of throttle. Each wheel should spin with the same force. If one wheel doesn't feel like it's getting power, you are losing half your power to that wheel when you are accelerating. You could be missing a pin from one of your CVDs.

Next thing is to take the slipper off and sand the pads to remove any glazing. glazed pads will kill your accelleration and if you are testing on grass, top speed will be much lower too.

Failing that, pull the pinion off and give the truck a push with your foot...a good firm push should get 10 feet or more before the truck stops rolling. If it stops in 5 feet, you have binding somewhere.

Check the 3 screws that hold each gearbox together..overtightening any one of them can bind up the gearbox or bind the slipper/clicker if the screw is sticking out too far
Check that you haven't overtightened your wheel nuts (if you are running a non-FT truck with plastic wheel hexes)

Check that your belt isn't too tight....

If you still have binding at that point, I would take the belt off and roll just the front on the ground, then compare that to rolling just the rear on the ground....that should give you an indication of which end is binding...or binding the worst.

Look for bent a bent CVD.
If the binding is in the rear, look at the axel carrier where the bearing holds the pin into the CVD. A friend of mine managed to put the truck together with the pin partially out, and it bound up that CVD against the bearing.

Next you might have to take the gearboxes apart...
Check that your idle gears and diffs are installed with the proper orientation.
Check that your diffs are properly assembled...if the cover isn't tightened down equally, the diff case will bind against the gearbox.