the grunt-thing singing is not at all a sludge thing. it’s more a common metal way of expression, but sludge is more related to “extreme rock” than metal and if you listen to pure sludge like Electric Wizard for example, there is very often only clean singing. For more stoner bands, it’s the same thing, there are a lot of clean-singing-only songs and instrumental songs too. And about post-rock/post-metal/shoegaze, those styles are very often purely instrumental so it’s pretty easy to avoid the grunt if it’s not your thing 
some examples :
Electric Wizard, pure sludge, some screaming but no grunt :
(if you evolve in the sludge world, you will always heard about Electric Wizard which is a very fundamental band)
Elder, psychedelic stoner, clean singing ot at least not grunt-ish at all :
Also if you like this musical universe, you might be interested by space rock ! and here is a band I absolutly love, named after the first man in space : Yuri Gagarin . if a song sounds like progressive/psychedelic rock (or stoner) but uses way to much phaser and rotary sounding things, it’s space rock
And also I would like to precise one thing : the sludge part is the slow, dark and kinda hypnotic rythm and ambience (it’s here with Elder but not at all with Yuri Gagarin, for exemple). The constantly evolving, progressive living structure with no clearly identified verse or chorus is not a sludge thing, it’s more the post-* (-rock, -metal …) part ; this particular aspect is some kind of natural evolution of psychedelic and progressive rock/metal.