99 Ranger won't stay in 4WD

I have a friend that is having problems with his 99 4X4 Ranger(6 banger,manual).Before winter he had new hubs put on because his 4WD didn't engage.Now he says that when he puts it in 4WD sometimes it will engage and when it does it only stays in for about 25ft just enough to pull out.He also said that it makes a hell of noise like a grinding,almost like something isn't engageing the whole way.Can anybody help on this one?

those are vacume hubs so it might be a prob with vacume lines or canister.they should lose vacume to engage. pull off the lines and cap them to eliminate this being the problem

Thanks,I'm tell him.