Knocking Noise when 4WD Engaged

[TL44]

1981 FJ40, all stock drive train, no lock out hubs. So we had a fresh snowfall overnight and I took my 40 out early this AM and in 4H when accelerating I hear a knocking noise coming from underneath. The 4WD is engaging and no noise when in 2WD. I have not looked at anything yet so any and all suggestions/advice will be greatly appreciated.

 

[Pighead]

Slide yourself underneath and see if your skidplate is interfering with the front driveshaft...

 

[71FFJ40]

Grab hold of the front drive shaft and see if you have any slop/Check u joints.

 

[TL44]

Grab hold of the front drive shaft and see if you have any slop/Check u joints.

Ok thanks,will do! What's about the normal amount of play?

 

[ibinazbass]

There should be zero play in your u-joints. With a knocking noise you'll typically see one (or more) bearing caps that have rust colored dust coming out of the seal too

 

[65swb45]

With the thinness of the factory skidplate on your year Cruiser, I think there’s a high probability that @Pighead guess is Occum’s razor.

 

[igotta40]

It’s precisely what my knocking sound was. I was impressed that such a slight contact made so much noise.

 

[TL44]

No play in the U-joints and the skid plate was not hitting. I can rotate the front drive shaft about an inch is all I noticed.

 

[shortroundj]

I'm having same issue and guessing it's my skidplate. Makes speed sensitive knocking noise under the floorboard passenger side under hard off the line acceleration when in 4wd with and without hubs locked and also with hubs locked but in 2wd - so basically any time driveshaft spinning but not load on driveshaft dependent ie if not driving front wheels. And guessing hard accel shifting weight back just enough so that it rubs. Can feel it vibrating in floorboard. But the skidplate doesn't look dented interestingly.