I have to pull the axles from my Dana 60. Are there any tips to ensuring the spacer doesnt drop off the pinions?

other than luck, not really.
just my personal experience. and yes, i have said bad words after looking into the tube[s] and seen the spacer[s] missing.......

The end play spacer should have the connecting pin peened over but most of them aren't peened
If one of the side parts pulls out and drops off into the center section you will probably have to pull the center section to retrieve the part that fell off, which is a perfect time to reinstalled the spacer and peen that soft center connecting pin

Actually may have found a better solution. My whole reason for pulling axles was because the shop that rebuilt the rear-end, put the brake backing plates on the wrong sides. However, there is a boss there which would allow me to drill the backing plates as installed and permit me to connect the parking brake. Not factory correct maybe but it isnt an oem show car anyway.