what is the best way to test diffirental
 

how do i know if diff is bad while it is still in car

Typically on a Camry unless the diff is run out of fluid it rarely fails.

The diff can make a howling or grinding noise.

Drain the diff fluid and check for metal debris.

its a good advice... I would try this some time.

The diff unit on the Gen 4 - four cyl has its own fill and drain. Changing the trans fluid does NOT replace the diff lube. I once had a moron at "Drippy Lube" tell me "you don't have a differential, you have a transaxle" after I asked then to change just the diff fluid (which is hard to get to) ( the trans was filled with Mobil One trans fluid at 5 bucks a quart and I sure didn't want THAT drained.) I had the Haynes with me and even after I showed the moron the listing for trans oil capacity AND a separate amount for the dif oil (both ATF) AND the photo of the diff dain and fill plugs he repeated "you don't have" over and over. I left. Noise from the diff can be reduced with new fluid, perhaps a gear oil with a higher "weight" (75-90) than ATF, but be sure the noise isn't from a CV joint going.

The differential, of course, allows the outside front wheel to turn slightly faster than the inside one in a turn while, driving BOTH when going straight, so the gears inside are always under some kind of load. It can make noise all the time or just in turns. Strange noises or steering feel in turns is usually CV joint related, though, unless you find your diff is EMPTY of fluid.