Rear diff - count how many times the rear driveshaft rotates for each wheel rotation by hand.
Front diff - i believe you can do the same thing by peaking through the diff plug at the ring and pinion teeth. Or, if the front diff is already bad just yank the axle stubs and look through there or bash the casing open with a sledgehammer and count the R&P.
I have replaced with brass and likely posted pics in here under username Jono
I used the 1/8" taper brass bits in 28 tpi starting at the block. Also used a 45° elbow so it gave path a downward dog clearing my mpfi intake, then maybe a shorted barbed nipple. I think may have been three short pieces at the block end
I think the importance of this is as an air bleed as there isn't much volume carried through.
I think it worked, may be on an engine yet to be installed
Front diff is part of the trans.
If you need a new diff you need a trans.
Let me ask though, how was it determined it needs a new front diff. That is extremely rare. Other causes like broken CV axle or bad wheel bearing can make bad sounds, and mimic front diff trouble.