Wow! I haven't seen a two spider diff in a savage in a really really long time!
Personally, I'd clean the ring and pinions really well, soap, water, elbow grease, then inspect them for wear. I have a feeling if you saw them next to a new set, you'd notice wear. Just look really close for any chipped edges on the ring/pinion.
Even then, I'd upgrade the diffs to 4-spider diffs or you will be back here soon replacing them anyway. Those older 2 spider diffs were not known for their durability. The old cast savage gears just in general weren't known for greatness.
This is the older 4 bevel upgrade:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXJLH6&P=7
Comes with longer machine thread screws as well. Screw them in slowly or you will overheat the threads and strip them out.
Also, make sure to check your input and output shaft bearings for the diff.
You may just have case slop from time considering the age those diffs have to be. Over time, the case flex's and bearings create slop where they sit. It only takes a hair of slop to make the teeth skip under pressure in a plastic case.
Pull off the big sun gears inside the diff (the larger gears in the cup) and see if the pins that they sit on are also ok.
Again, I'd bite the bullet and just pick up a pair of XL diffs off ebay. $60 is better than $25-$30 now for new bearings/bevel gears then $60 later when the ring/pinions die on you. That's why I also suggested spending a bit more now with alloy cases, new diffs off ebay (with new bearings) and proper shimming. Might seem like a lot of money, but a bit of preventive repair now keeps you going much longer and worrying about other things... like center axles.
