Scotty, I have an interesting issue going on. 

I have a 1999 Chevy Silverado with 344,000 miles on it. Ever since I got the truck from my father (306,000 miles) its had this kick to it going up hill. It hasn't gotten any worse or any better in all the miles I've put on it. What it does though, whenever I am going slower (Usually 30-40 mph and sometimes around 55mph) and uphill at the same time and just feathering the throttle, there's this slight kick and I hear it sometimes make a slight clunking noise from the front of the truck. If I downshift, it goes away and runs normal. it also does not do it at all in First and Second gears and does not do it at highway speeds. 

I recently had my spark plugs changed, thinking it was a possible misfire (And they needed to go anyways) and the kick went away for a full week. Well it's come back again, but not any worse than before. I really don't know what is causing this. My only other thought was maybe a torque converter? But I'm not an expert. There is a slight rpm change when it kicks as well. Any help would be appreciated bigly!

Thanks