What is wrong with your diff?
If it is just crusty red looking on the spline that may be just the dried up factory moly lube.
Forum members recommend re lubing with the correct High 65+% Moly every tyre change. I have posted the details before and have a tub of it here somewhere, grey thick goo!
To check for damage clean off the splines and examine the wear. Note if major wear the splines on the prop shaft may be toast too. Take photos close up of cleaned and experts like
@Mittzy,
@ozrider and many others can assess them for you. If you search threads here on the forum there are photos of good , bad and usable to allow comparison.
I'll keep an eye out.
BE AWARE that the later Black painted 2010 ABS Roadster ones (T1275000 - DRIVE UNITFINAL
87D) will not fit an early series Rocket III. like your 2007 Classic with the Silver painted/alloy look (T1274000 - DRIVE UNIT FINAL S).
The early ones have a smaller diameter spline on the prop shaft apparently but if you change the prop shaft as well you can use the black Roadster ones .
They changed the angle of the drive shaft mount for the Roadster version to fit that prop shaft I believe.
(Note the 87 degree reference)
The Rocket III Touring used the same Silver painted/alloy diff (T1272050 - DRIVE UNITFINAL90) from 2008 -2018 despite adding ABS in 2013. (Note the 90 degree reference).
Browse OEM Parts for Triumph Rocket III Touring 0 Final Drive Unit
www.hcmc.com.au
T1272050 - DRIVE UNITFINAL90
Reference
1
Part Number
T1272050
Description
DRIVE UNITFINAL90
Price
3044.30
********************************************************************************
I have a spare 2010 Roadster one (T1275000) that won't fit your bike.
Browse OEM Parts for Triumph Rocket III, Classic & Roadster 0 Final Drive Unit
www.hcmc.com.au
T1275000 - DRIVE UNITFINAL87D
Reference
1
Part Number
T1275000
Description
DRIVE UNITFINAL87D
Notes
Roadster only
Price
3044.30
T1274000 - DRIVE UNIT FINAL S
Reference
1
Part Number
T1274000
Description
DRIVE UNIT FINAL S
Notes
Rocket III & Classic
Price
3044.30