99 3.4 2wd manual clunking noise when shifting

Hey all,
Hoping for some ideas. I got a definite clunk when shifting.engine/tranny mounts look good. U joints look good. There is an added upper inverted aftermarket leaf spring (came with when I bought the truck). The mounting brackets (the bottoms of which have 'rollers') do move around but it doesn't seem like this could produce such a loud clunk. I can reproduce this sound with the axle up on jack stands and lugging the engine before stepping on the gas. I've been to two shops. One said U joints are bad tranny is fine. The second said tranny is bad U joints are fine. Any help appreciated. My wife won't drive it like this.