With my own two hands I've taken a mazda tranny out of a 4.0 v6 Navaho, a rebadged ford explorer, and got some rebuild parts from a wrecked ranger with the same engine/tranny.

The main failure point in the fm/km145 is the input shaft spacer plate for the bearing. It gets pounded out and lets the bearing and shaft wobble. Almost every time I have exposed the input shaft on this tranny, you can wobble the shaft a bunch if the tranny has more than 100k on it. The rest of the tranny seems to hold up pretty well. It was used behind the starquest 2.6T and had to be beefed up in the above area to handle the higher torque. Even with that, the engine was confined to a lower boost level in the lower gears...

Here is the best resource for understanding the nightmare that is the Ranger Transmission. http://www.therangerstation.com/tech_library/ManualTransmission.shtml

The transmission you are referring to is a Mazda M5OD transmission which was matted 4.0L V6 like you stated. I was referring to the older Toyo Kogo (TK-5) 14 bolt trans. The FM145/46 was found behind most Rangers/Bronco II with the 2.3L Diesel, 2.8L V6 and 2.9L V6 engines.

A good rule of thumb to go by is:

If you have a Ranger newer than 1988 with a manual transmission AND DO NOT HAVE A 2.9 ENGINE you have a MAZDA M5OD-R1.

If you have an 1988-1992 Ranger or Bronco II (1988-1990) with a manual transmission and 2.9 engine you can still have a Mazda transmission but a Mitsubishi is far more likely.
The Mitsubishi has a bottom cover, a Mazda does not.

1983-1987 2wd Rangers and Aerostars (all engines) with Manual trans will be either a 4-speed or 5-speed "Toyo-Koygo" transmission. (Note: Aerostar 5sp only)

1983-1984 4x4 Rangers/Bronco II's (all engines) with Manual transmissions will be either a 4-speed or 5-speed "Toyo-Koygo" transmission.

1985-1987 4x4 (any engine) can be EITHER a Toyo-Koygo 5-speed (aka "T-K" or "Ranger-5") OR it can be a Mitsubishi FM145 5-speed manual (this transmission is 4x4 ONLY)

Toyo-Koygo's are more likely to be found in 4cyl 4x4 Rangers. Bronco II's are more likely
to be Mitsubishi FM145's

1988-1992 Rangers (4x4) / Bronco II (4x4 & 2wd) 2.9 engine can be;
1) Mitsubishi FM146 (2.9 4x4 ONLY)
2) Mazda M5OD-R1 (4x4 version)

1988-1992 Rangers 2wd equipped with the 2.9 engine there were both;
1) Mazda built M5OD-R1 (2wd Version)
2) Mitsubishi FM132 (2.9 2wd ONLY)

1988-current production Rangers with 4cyl engines both 2wd and 4x4 (including the 2.0 (carb), 2.3, 2.5 and (new) 2.3 DOHC will be a Mazda M5OD-R1. This is the only manual transmission factory installed behind four cylinder engines in Rangers after the beginning of the 1988 model year.

1988-1995 Aerostars with manual transmissions will have Mazda M5OD-R1 transmissions.

1991-current Rangers with the 3.0 engine:
The Mazda M5OD-R1 is the only manual transmission used in Rangers with this engine.

1991-2000 Ranger/Explorer with the 4.0OHV engine:
The Mazda M5OD-R1 is the only manual transmission used in Ranger/Explorer with this engine.

2001-on Ranger (1997-on in Explorers with the 4.0 SOHC engine):
the Mazda M5OD-R1HD is the only manual trans used in Rangers/Explorer with this engine.

NOTE: on Bronco II's ALL versions are equipped with 4x4 type transmissions,
because the 2wd Bronco II's use a "dummy" transfer case. I don't know why Ford did it this way, but that's the way they did it.

May I ask what in particular is different in the starquest application? Guys over there are under the impression that the truck tranny is the stronger one. Yet over here the consensus is the opposite. Every once in a while I see a post of someone trying to build a hybrid using the "stronger" components of the oposite vehical they have. I know the car uses a torque tube so the fifth gear gets lubed with tranny gear oil while the truck's fifth gets lubed with transfer case gear oil. Otherwise am not sure what else is different. Thanks.

Also, besides the input bearing, I hear that the case itself flexes a lot adding to the problem. Hence a tuff pan supposedly helps. Havnt seen proof but for $120 why not. Also static electricity pits the gears, particularly fifth. A couple rock auto ground straps are said to help.