you will need to reuse the center block however since the front diff won't have it...this keeps your rear axles/c clips in place.
68-newer diffs from a 40-45-55-60-62 will all have the same side gear splines and are interchangable.
Make sure you check gear ratios....your 75 should be 4.11.
Driveshaft bolt patterns vary some....your input flange is coarse spline and will interchange with any other coarse spline diff, but tha'ts only from 1977-older. If you end up with a newer fine spline pinion, you'll likely need to source a different flange for the d-shaft.
Yes you can get differentials from any 40,55, 60, or 62(i am assuming you are in the US) and they will fit in your truck.
what you need to know is
4spd or 3 spd diffs (if you get one or the other you will need to change your pinion flange OR you can take the appropriate drive shaft and swap that in on the end you need it(i.e. if you have a 4spd and you get a 3 spd diff you will either need to swap pinion flanges OR get a 3 spd drive shaft end (which will fit with the other end of your 4spd shaft (making lots of sense?)
Gear ratio...40s,55s,60s(up to 85) and 62s all have 4.11 gears...i think 85-88 fj60s had 3.73s (yes no probably wrond on my years there, count the teethe on both diffs if they are the same you are OK.
last yup you can put the front diff in the rear and the rear diff in the front....I have 2 spare 3rds from a 3 spd..interested? PM me or email me
Canadian vehicles started getting 3.70's in february 1976. (37 tooth pinion, 10 tooth ring). They went back to 4.11's in January 1979. US models went to 3.70's in January 1979. All remaining US 40's and 60's got 3.70's. FJ62's got 4.11's.