Ford Repair: Differential, bronco 4x4, ford bronco

Question
I have a 1990 ford bronco that i purchased from my cousin that has a set of 33, 12.50 tires on it.  My transmission or my differintal at highway speeds makes a ungodly clunk noise when it downshifts from overdrive.  it also bucks the entire truck and im worried about destroying either the differential or the transmission.  what i would like to know is if i change the gear ratio would that help get rid of the bucking and the noise.  the gears from what i am told were never changed so that led me to think that they would need to be changed.  so please let me know what you think as to if i should change the gear ratio.  also how do i figure out what gear ratio is in my truck.  Also what is more cost effective and works better a limited slip or a possie differential.  I do off roading and towing if that helps any.  thank you....Jared Piper

Answer
Jared, the Bronco 4X4 noises may be a combination of clearances that add up to a lot of driveline backlash.  On that vehicle, a lot is normal, but some things will wear with age and use, such as universal joints.
I would start with a close inspection of the driveshaft U-joints, and replace any that are worn.  Inspect the tires, too - they must be the same size and preferably about the same wear on them.  When you go into 4WD, all the tires must be about the same total diameter or the driveline will be working against itself.
As for rear end ratios and Limited Slip vs. PosiTrac, the front and rear ratios must be the same.  Changing one requires that the other be changed, so that gets very expensive.  The limited slip is used by Ford to indicate that clutches or cones are inside the diff to prevent wheel spin.... GM used the term PosiTrac.  Both are essentially the same thing.
The gear ratio is stamped on a tag that is bolted to the cover of the rear end... if it's missing, you can raise the rear end and turn the driveshaft by hand - count the turns that the driveshaft makes as one wheel is rotating one time.. that should give you a close approximation of the ratio (if the drive shaft turns just ove 4 times, the ratio is probably a 4.11:1  If it turns just under 4 times, you probably have a 3.73:1)
Hope this helps....
Clay