There is a problem using car rearends in a truck, The truck will no longer be able to cary any weight or load. True you can get higher gears, but you lose the load rating of your truck. There is a reson the factory put those big rear ends with the huge bearings in there in the first place... to cary weight. The type of swap mentioned above is more in the line of acceptability in a 1/2 ton truck, but not a 3/4 ton truck that may be asked to work.
