Thumping in front end in 4WD

Well we have quite a bit of snow here in the Seattle area and I put the '85 K5 Blazer into 4WD of which it hasn't been in for a few months at least unless pulling out my 9000 lb boat out of the water. Anyhow, when driving
straight and getting over 25-30 mph, I get quite a thump in the front end somewhere about every so often. The 4WD is engaged because I can feel it so I know that isn't the issue. I have Warn manual hubs and then shift on the floor (which is factory). I haven't done any service recently to the front axle so not sure what is going on there. Anyone have any clues as to what this might be? Didn't notice this thump when turning, just when going straight and over 25-30mph. Thanks.
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Also want to add that this when I engage into 4WD Hi. The hubs can be locked and it not happen. Thought this might help in giving me some feedback. Thanks and hope to hear something soon.

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Had a similar problem with my 87 K5. One of the spider gears had lost a few teeth. One was broken in half and lay in the bottom of the housing.
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Is this expensive to fix? Would probably take it to a shop as I haven't done much with axles. Anyone else have any other ideas?

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Mark, sorry for the delay, but life was calling =) If just the spider gears were involved and there is no ring or pinion damage it can indeed be a cheap fix. My cost was about 40 bucks total. 30 dollars for a used spider set at the local u-pull-it and 10 dollars for incidentals. Gear oil, blue goop, and band-aids. Be sure to wipe the case dry to get all of the broken pieces out. Look around in there carefully, make sure there is no debris left in the case or you may be doing it again. I'm almost betting that you inadvertently let the case go dry, or nearly so. If you have never done anything like this before, offer a knowledgeable friend a six-pack to give you advice while you are doing the work. A first timer should be able to do it in a few hours, start to finish. Good Luck JR
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Can't anyone else offer anymore on this? Snowman?
Seems this group used to be more active in providing a variety of responses from knowledgeable folks. What happened here?

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Sounds like it's time to give me your truck so I can combine you 85' K5 and mine into 1 better ride..... gimme a call if you decide you want to give me your truck... :0)
I think it's the spiders... sounds like it's time for a nice locker... And no, front end work 'taint cheap... nutin' be....
-The Lonely Grease Monkey 1985' K5 305CUI TH700R4 NP208 KJ's successor
"Ignorance is preferable to error; and he is less remote from the truth who believes nothing, then he who believes what is a wrong." - Thomas Jefferson
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Well I did find out where the problem is coming from. It is coming from the hubs and not the axle like first thought.. I found this out because I unlocked the hubs and then shifted into 4WD and the thumping was no longer there. Maybe the hubs are just dirty or have ice in them or something. It has been pretty cold here. What you all think of that? Thanks.
BTW, Lonely Monkey, my truck rocks, just occasional irritations. Heck, it can pull 9000 lbs and idle up the muddiest hill better than a jeep. Does great in the snow too. Do want new tires though.

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Mark, you may or may not have found the problem. If you unlocked the hubs and shifted into 4x4 the spiders weren't doing anything. Try locking the hubs and drive in 2x4. See if the thumping comes back. Please let us know what you find. JR
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It doesn't do it when in 2X4. Just when in 4X4. The last trick I tried upon a suggestion from a mechanic I go to sometimes. He said he is willing to bet it is something to do with the hubs. Maybe dirty or dry hubs/ What are your thoughts? Thanks.

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