You need to further diagnose the drive line. Wheels off of the ground, start with the front: inspect wheel hubs for excessive movement, same with the inner CV axle and differential carrier interface. Spin the wheels and listen to the carrier, unbolt the CV axle from the diff flange and work its range of motion looking & listening for suspicious noise.
Then you inspect the driveshafts: best way to do so is remove the pinion-end and move the U-joint throughout its range of motion, play/movement within the bearing caps, rust, missing needles, etc. same for the rear DS. By completely removing each driveshaft the slip yoke can be greased before re-install.
Examine the rear hubs with wheels off of the ground, check for in/out & up/down movement of outer axle, spin the wheels with a stethoscope on the carrier housing, etc.
You would be needlessly throwing parts at a problem of which you don't entirely know the nature or origin. 1-person opinion doesn't make it so, please thoroughly run through a checklist from front of vehicle to the back. In doing so, anything that seems suspect can be noted and dealt with simultaneous of the larger problem you are seeking, or at a later date. Possibly all parts gathered from the recycler so that you aren't making a 2nd trip.
Whenever the components were drained of oil, said oil should have been collected in a clear can & inspected for metal particulate. If you know the condition & contents of oil, replacements you can be certain of depending on your findings. Without oil contamination present, you are diagnosing something unrelated to the differential proper, like an outer wheel bearing.
Be certain the problem doesn't originate from the front end by gathering some Intel via Google & forum search along with the suggestions I have advised, then progressively work backwards to the vehicle rear. You are more than welcome, encouraged even, to share your findings and ask questions. Please take photos, as it is easy to attach with the posting. Pics goa long way trying to diagnose over the web. Also, video can be recorded & uploaded to YouTube; then just copy/paste the video link into your posting.
If you really want to isolate the pinion, I suggest removing the drive shaft for starters, raise the back of the truck & put it through the paces.
http://www.differentials.com/diagnosing-differential-problems/
Any problems or failures emminating from the diff' will reveal itself with noises and what can be felt, not necessarily vehicle side/side movement @speed which is more closely associated with an outer bearing.
The front axle housing output has a bearing/seal combo just behind the CV axle attach flange. If the seals have been leaking for a long time and the diff was low on fill-level, the bearings could have suffered. Always inspect the oil fill level of any component before draining as this is always part of diagnosis. If you have info regarding this, and the orig oil conditions upon draining, please share.
Personally, I wouldn't even consider swapping/replacing the axle assembly before removing the diff cover and examining the carrier component for damage & imminent failure. Without any evidence of dire situation.
http://www.differentials.com/technical-help-2/failure-analysis/
