Transmission suddenly no reverse

[QX4by4]

Alright, so I have a 99 QX4 and the thing decided overnight that it will not go in reverse. Shift lever functions properly, and it will drive forwards but when you try to reverse it is like the engine has a load on it but goes nowhere. The strange thing is that even when in neutral it can be pushed forward but not reverse. The rear wheels just lock up. I found a rear end at a junk yard for cheap so I am trying that this weekend. Hell of a lot cheaper than a transmission if that is the problem. Regardless, has anyone else had this happen? If it turns out that the trans is shot can a pathy trans be used instead? The junker the rear end is coming off of is a 99 pathy. I have 160000 miles on the Q and I intend to put many more on the clock I just don't want to spend 3000 to rebuild the transmission. Any suggestions would be appreciated.

[QX4by4]

Update. Definitely the trans or transfer case. Something broke loose inside it. The new rear and trailing arms got rid of the death wobble but still no reverse. Stupid automatic. Wish they had made the Q with a standard shift transmission. Guess I will update if I ever find a 4 wd transmission that will fit.

[Buzzman]

Just thinking, if you're going to replace the rear diff, try disconnecting the driveshaft first and see if you can push it backwards.
As for the difference between a Pathfinder and a QX4, it's all in the body, trim and cosmetics.
The drivetrain is exactly the same in both.
A Pathfinder transmission will work in a Q, and vise versa.

[Slumpert]

Humm for some reason I thought the qx was electric switched transfer vs manaul throw on pathfinders