Ok, my '99 4.6 4x4 rolled 100K miles a couple weeks ago. For about the past 4000 miles, when I am in overdrive and am starting up an incline, if I try to baby the throttle to just make it up the hill or to just delay shifting out of overdrive as long as possible, I feel a clunk (sometimes more than one) right under the truck seemingly from the tranny. It doesn't automatically slip back into 4th gear, just clunk. If i give it some more gas, it shifts out of overdrive as normal. Right after this started happening, I drained all tranny fluid including TC and changed the filter and it's still doing it now. Any suggestions would be appreciated. Thanks in advance.

 

It might be a missfire while under load. I replaced the plugs and coil packs and it went away, just my .02

 

yes, that is possible but i just changed the plugs about a month before it started happening. Thanks though