Loud thunk every 100 feet in 4wd

[redjmp]

My 97 xlt 4.0 seems to engage into 4wd just fine in the snow but if I'm driving on a wet road and engage 4wd, I get a very loud thud every 100 feet or so and the cab visible seems to drop and inch or 3...
It really feels like it is the drivers side making the noise...
Any ideas folks?

 

[scotts90ranger]

that's the drivetrain loading up then unloading when there's enough force built up for something to give (probably a tire spinning), don't use 4x4 if the road is just wet...