Like Mike said...Quadratec for most stuff...Gears, etc...Randy's ring&pinion...Driveshafts....Tom Woods....engine performance...Hesco.
JC Whitney has a lot of stuff...Rosser Think Jeep...mainly caters to short Jeeps...but they are good folks. You can feel safe (in my experience) ordering from any of the above and I don't pass out praise lightly.
I'm sure there are many more fine shops out there that I haven't dealt with yet...if anybody has some names, I would like to know also.
Steve