HELP! 93 rodeo 4wd engaging in and out of 4wd
 

hello i bought my ex a 93 rodeo and when u drive it it makes a bad growling and popping sound when u slow way down, this comes and goes if u put it in 4wd it goes away if u put it in 2wd it goes away put then comes back i think its trying to engage in and out of 4wd. is this the linkage? or is it vacuum operated? is it the hubs? i think it just might be the shifter linkage is loose and when u hit a bump it half way in 4wd half way out is what it sounds like. anyone have any suggestions? thanks