98 4.0 4x4 thumping/shuddering under acceleration.....help please.

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hey guys -

I have a issue and its driving me crazy not knowing what it is. it seems intermittent and by no means am I a mechanic but when I have one ride with me, of course the problem won't show up.

I recently purchased a 98 XLT sohc 4x4 (auto - 4lo - 4 hi). two owner truck with 72,000 original miles. never had a receiver hitch or ball on it, so its never pulled anything. I know these transmissions are not known for reliability but I thought I felt a slight shudder during normal acceleration. not bad at all, just slight. shifts positive, doesn't slip. just a little vibration between 35-45 mph. also just put new tires on it and I skimped a bit there so im not entirely sure if maybe the vibration could be tires. I had a local transmission shop drive the truck to hopefully ease my mind about the shudder ( or at least what I think I feel ). he told me everything felt as it should and not to worry about it. prior to this the fluid level was checked and the fluid did not smell or look burnt. I did however add a bottle of lubegard (red) a few weeks ago. this is the only additive I have researched that is acceptable. the fluid is in the crosshatch while idling hot.

on to my real problem....

I can live with the slight vibration but two days ago I accelerated very aggressively to get out of someone else's way. it felt like someone was under the truck banging the floorboard with a mallet. it sounded awful. thud thud thud thud. the truck never slipped but it felt violent as it vibrated. it will not do this all the time but again, it done it this morning just as I was testing to see if it would. from a dead stop only. im really worried its something like transfer case or the trans. ONLY does it from a stand still and HARD acceleration. no lights are coming on the dash. I am probably not doing a very good job explaining this. to me, it feels like something is trying to engage but its engaging then disengaging over and over. also worried it is the 4x4 system trying to kick in - although the back tires are not losing traction when this happens.

just really worried and hoping its something relatively inexpensive.

thanks for any and all help.

 

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still trying to figure out how to describe this thudding/thumping sound....


maybe similar to that of rolling over very rough rumble strips ??

 

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removed the front driveshaft u-joint and it moves freely. I assume it is good. while leaving the front drive shaft connected to the transfer case but disconnected from the diff, I can swivel the shaft and there is hard spot. not terrible but noticeable. this looks like a CV type setup. could this be the issue?


also my other thoughts....

under hard acceleration from a stand still, even tho I am not slipping the tires, could the truck be trying to engage the auto 4x4 system? maybe not engaging fully but attempting to, causing the vibrations and thumps under the front of the truck.