Truck is F150 - 351, 5.8 liter, automatic. On gentle acceleration, there is a very noticeable thunk, clunk, that you can feel, between when its upshifiting from 1 to 2 gear. Where should I look? or check?

 

what year and transmission?

 

1996 I do not know what transmission, how do I tell?

 

I have a 96 with a 302. Chances are very good that you have an E4OD. Very reliable tranny. I have the same clunking noise. Check your fluid and flush if necessary.

 

Try looking at the Universal joints (located at each end of the driveshaft). Get under the truck while it's in neutral (make sure you've got the E-brake on so you dont die) And then shake and pull up and down on the driveshaft. If it has any play in it, your going to need to change them. Let us know what happens

Jim