I have an 89 4runner. Took it out 4xing and mudding a couple times and everything seemed good. Next time I took it out it started making infrequent grinding noises in 4h. Was only in high at first, now it does it in low as well. When it starts I take my foot off the gas and it stops, then it's good for a few mins and does it again.

After the first time it did it I didn't use the 4x at all for about 2 months, then at christmas we used it and it was fine for a few short trips before it started again.

Manual locking hubs, locks in no problem. Makes the noise in 4h, 4l and while the hubs are locked in but not in 4x4.

Any suggestions?

All oils are full, t-case, tranny and both diffs, rear diff looks a little dirty but the noise is definitely coming from the front end.

Haynes manual pretty much points to front diff or axle, could it be wheel bearings, or in need of grease in the hubs?

sounds like binding grind to me, 4wd should only be used on snow/dirt
sont use it on dry pavment it is really not good for it and can cause damage like you have.
look at the t case fluid and tranny fluid im betting its old and lost its shock value. wich means the insulation it would have agianst grinding and shock is gone.

I had issues last year where one of my manual hubs would occasionally not lock, and it would grind, as the axle was spinning opposite the wheel. It may be worth jacking up the front and making sure that they are locking.

I agree with the dingman, it might be a hub not engaging, I had autos on my 88 and it would grind in 4wheel, but after I installed manual hubs it's been fine. Also check your CV boots and see if they have any cracks, or holes which might have let dirt, water, salt, etc. get into the joint, and the crack might have allowed for the grease to come out. Good Luck

Just put the front end in the air, lock the hubs, and put in 4HI and 4LO and turn the wheels by hand. You should be able to locate the source by listening closley.

BTW, what happens when the hubs are locked and it's in 2WD?

Hubs locked and in 2hi will grind the same way. Hubs not locked and in 2 hi is fine.

It doesn't do it all the time, usually takes about a half hour in 4x before the grinding starts, but then it will do it every couple minutes. As soon as it starts I release the gas and it stops.

Only time I used the 4wheel on the road was on the way home from christmas when it was snowing and I nearly slid off the road. Having the crappy mudding tires I have didn't help anything.

Should I replace all the gear oil? Tranny, T-case and both diffs? It is a 5 speed if I didn't mention that already.

I was wondering if there is a grease nipple to do with the hubs. I removed the drivers side hub but don't really know what to look for. It was greasy but maybe could use more?

This seems logical to me. Would a change in speed help it engage?

Maybe. Think about this:

When you're accelerating, and only one wheel is locked, that wheel is likely spinning just a tad faster than the driveshaft, thus spinning the opposite axle backwards, causing grinding of the hub. When you slow down (with 4WD engaged), the axle may become the faster spinning object, then reversing the axle.

I'm not an expert on what goes on there, but I know that one time I had my 4WD engaged, and only one hub locked, and it was grinding. Once I slowed to about 25 MPH, it popped in place, but it was quite a jolt, and I'm surprised that it didn't break anything in the hub.

Jack it up, spin the wheels. Make sure to lock the hubs BEFORE you jack it up. When my hub was sticking, it would only do it when it was on the ground. My work-around was to jack the front before locking the hubs... Quite a pain when we get big snowfalls...

Somehow the hub works fine this year... Not sure what changed.