So a good deal popped up on craigslist for a 8" 3rd member out of an '89 pickup, but I'm unsure if I can use it in my '77?

Are they compatible?

 

The 8" 3rd member isn't compatible with the 7.5" housing. You could probably swap in the whole axle assy with a little fab though.

 

My truck has an 8", and I bought an 8" third member... I was unsure if the newer diff would work in my older axle.

 

Compare pinion flange its the only change iirc. Maybe someone more knowledgeable will chime in.

 

Early (pre 84) flanges have 8 mm bolts and a more rectangular pattern. Late (post 84) flanges have 10 mm bolts and a squarish pattern. Swapping flanges is easy or you can simply drill the early pattern into the late flange with no issue.