Since there has been a lot of talk past & present on re-gearing the FJ (along with 05+ Tacomas) I thought I'd make its own thread with all resources posted here. I had some info posted in an older thread, but we have found some updates since then and I wanted to have everything in one clean thread.

FRONT DIFFERENTIAL
The front differential is an 8" mid pinion IFS that uses a gear not available through the common aftermarket. It is the same as the Non-US Toyota Prado front differential. The only current ratios available are 4.56 from Toyota OEM Japan and 4.88 from Mossiero in Italy. All Pro has 4.56 OEM Toyota Gears in stock, and 4.88 on the way in.
Front install kits are also not available from the common aftermarket. It uses the rear 86-95 V6 pinion bearings, but everything else is unique to the FJ/Prado. All Pro has gathered the parts together and can assemble a kit for you by special order, and have preassembled kits in the works. No solid spacers are available for the front differential
One last thing - there is a case break on the front differential. This means there are 2 different cases, or differential offsets: one that fits 3.73 and down, and the other that fits 3.91 and up. So if you have an Automatic FJ with 3.73 gears and want to go to a 3.91, 4.56 or 4.88 ratio, you need an open diff from a stick shift wth the factory 3.91, or you can change to an aftermarket diff like an ARB and get yourself some extra traction at the same time.

REAR DIFFERENTIAL
The rear end uses the same 8" gear as the 86-95 V6 Rear, and 95.5-07 Tacoma TRD E-Locker. Here's the catch, Toyota changed the pinion spline count from 27 to 30 when the Tacoma began in 1995.5. Now, the aftermarket gears that will fit in the FJ & Tacomas are ONLY made in 27 spline. Why not use the Prado rear like the front you ask? The Prado is like the FJ80 series LandCruiser diff, 9.25", NOT 8", so it wont fit. NO aftermarket gear manufacturer currently makes a 30 spline 8" V6 gear set, but 4WP Genuine Gear is working on it, with probably 8-10 months wait. They will be manufactured in China.
So is it safe to use the older 27 spline V6 gears? Yes. The OEM FJ gears that are breaking are doing so at the tooth, not the pinion. There is NO DIFFERENCE in pinion tooth strength between the 27 & 30 spline gears, if one would break, so will the other. Guys in the mini trucks have been running 200:1 & lower gearing for years with success, and with 37"+ tires sizes, so as long as you buy a quality set of gears (Precision, Yukon) you should have no more problem with the 27 spline than with the 30's, because remember, its the teeth that are typically breaking, not the shaft at the spline.
So what do you need to do to make them work? Install them correctly with quality bearings, and we STRONGLY suggest using a solid spacer (Crush Sleeve Eliminator) in the rear instead of a crush sleeve - $29 of cheap insurance - and a new pinion flange. The pinion flange must be 27 spline to match the gear, All Pro stocks and sells the correct pinion flange to do this installation when used with the V6 or E-Locker master installation kits. Install kits are simple, if you have an E-Locker equipped FJ, you simply buy a Tacoma E-Locker master install kit. If you have a non E-Locker rear, you need the 86-95 V6 install kit.

I hope that clears up the confusion on regearing the FJ - we did hours of research both with tearing apart diffs in the shop & dozens of calls to Toyota to get these facts. The bottom line is you CAN re-gear your FJ. PM me if you have any questions, I'd be happy to explain anything thats still foggy.

Terry
All Pro Off Road
951-658-7077

Shopping list for 4.56 gear change:

Front Gears: All Pro 7107-AP-456
Front Installation Kit: All Pro
Front Locker 3.91+ Ratio: ARB RD111 (required only if FJ is an automatic)

Rear Gears: Yukon YG-TV6456
Rear Installation Kit: Yukon YUK-TKTACOMALOC for e-locker, YUK-YKTV6 for non e-locker.
Rear Crush Sleeve Eliminators: All Pro 7501-AP (E-Locker applications ONLY)
Rear Pinion Flange: All Pro 7600-AP-FJ