This problem occurs at slow speeds right? This is not a sign of failing viscous coupling because the viscous coupling WILL do its job when the tire slips aka on a slippery road and very little traction. However, in a parking lot you will be going at such low speeds that the tire will have more than enough traction so that there will be no slipping aka the viscous coupling will still give equal power to each tire. So the bucking is caused by the truning front tires pulling the way they are pulling ang the back ones pusching forward causing a kinda push and pull thing but at high speed you do this and the front tires will lose traction causing a slip and the viscous coupling will do its job transfering power and less power to the front tires gives the backs more force and they will win this "tug o' war". At higher speeds stocker is right maybe even in the parking lot, there could be a problem but mine only happens at VERY low speeds, so i would get it checked out either way and see what they say.
-'98 5.9 Grand Cherokee-