Hello,
I know that the T-Maxx is slowly dying but there are still a good amount of people out there that are building them up.

FLM is one of the few places that still carry a full line of T-Maxx parts, but they are starting to discontinue some of them.
I myself recently built a full FLM T-Maxx and had to do quite a bit of searching to find part numbers for the differentials. I see a lot of threads here asking what combinations can be used in there Hybrid bulks as well.
I don't know if there is a thread listing part numbers or not so I'm sorry if there is and this ends up being a double...
From my searching this is a small list of differentials that can be used in the FLM Hybrid bulks for the T-Maxx and I am sure for the Savage as well. It is not complete but it is enough to get someone what they need! Feel free to correct any mistakes you may see.
Caster Racing PRO Buggy - 8x16x5MM Bearings - Pinions stepped to 5mm
ZX-0114 Complete Diff
ZX-0121-PRO Crown Gear Set (43t &13T)
ZX-0061-PRO Pinion Gear (13T)
ZX-0060-PRO Diff Gear (43T)
ZX-0119 Buggy Steel Spiral Bevel Gear (43T) - FLM uses
ZX-0120 Buggy Steel Sprial Pinion (10T) - FLM uses
Losi LST2 Differential
LOSB3529 Complete Diff
LOSB3534 Ring and Pinion set
Ofna Hyper 7 Spiders (8 gear)
OFN87343 Complete Diff
OFN19028 Spider Gear
Ofna Hyper 7 TQ Differential
OH15 Complete Diff w/bulk (Can find part # for just the diff)
19001 Bevel Gear (43T)
19028 Pinion Gear (13T)
19002 Cup Gears
19004 Diff Cup
Ofna 9.5 Differential
OFN40007 Complete Diff
OFN49032 Bevel Gear (43T)
OFN49012 Pinion Gear (13T)
OFN49001 Cup Gears (steel)
OFN40003 Diff Cup
OFN40009 O-Ring
HPI Savage Differential
HPI86127 Plastic Housing Complete Diff (43T)
HPI86826 Alloy Housing Complete Diff (43T)
HPI86031 Pinion Gear (13T)
HPI86030 Bevel Gear (43T)