ka24e timing chaim guide toast.

How difficult is it to replace the guides? '93 Nissan d21 4wd  

You probably need the whole timing kit, gears, chain, guides, seals, ect. When these engines have a timing chain verses a timing belt it means it has interference heads. That means if the engine loses a timing chain or the guides get so loose they cant keep the chain tight with the timing gears, the valves and pistons slam into each other and bend most or all the valves. The timing job is not an easy one. If you hav'nt done it before, don't, it's not worth the time, money and trouble you will go through. Have a pro do it. If it gets too loose it's gonna skip gears and then it's done. Head job or salvage engine will be in your near future. So if you are still driving it and the chain is getting too loose, shut er down and get it fixed.