zebelkhan said:
My 2001 Toyota 4x4's 4WD system could do up to 65 mph when engaged. With transfer case locked (even power front to back and side to side) could do up to 20 mph. I always wondered why I would need to go that fast in 4x4 mode...
Ahh yes, but if you had the TRD Limited Slip Differential, you would have found that it limited you to 15 mph.
The reason is this (from an avid offroader):
Locking your AWD or, if you have a 4wd with LSD, will cause drivetrain binding. In other words, if you go on dry pavement, lock the AWD/LSD, and then turn sharply, the outer wheels spin faster than the inner wheels. When the wheels are locked together, this causes binding. This can eventually cause the drivetrain to break. Very $$$.
Also, locking your AWD or LSD really only needs to be done when you are stuck, or at a high risk of being stuck. In mud, snow, sand, etc, gassing it as hard as you can only spins the tires and digs you deeper (I learned that the hard way). You have to slowly finesse your way out. For this reason, there is no need to have the AWD/LSD lock at high speeds, since if you need to use it, you should be going very slowly anyhow.
Also, if you are stuck, lower your air pressure to 20 or so PSI. Back on my Tacoma, I used to drop it doen to 10PSI, I had BFG Mud Terrains which worked great at low pressure.