Mystery Thump in Chevy Tracker

Recently purchased a '99 Chevy 4 door Tracker (4wd) with 60k. When the car shifts from 1st to 2nd (automatic) under medium to heavy acceleration, I can hear a ‘thump’ coming from the back of the car. It does not affect performance. It will not occur under a slow/gentle acceleration or during any other shift.

The U-joints were replaced and the mechanic checked everything in the back end (tranny mounts, bushings, exhaust…). He saw nothing out of order and has admitted he is stumped. I’ve owned 3 Trackers and never experienced this before. Any thoughts on what it might be or what else I should check?

Spend a few minutes grabbing everything in the back of the Tracker and shaking it real hard. The last time I heard a complaint like this it turned out to be a loose gas tank. Is your spare tire tight?

The spare tire is on nice & tight. Humm, the mechanic didn’t mention he checked the tank (might not have gotten a good grip on it 'cause it has a guard surrounding it). I’ll check it all out. Thank you very much for the suggestion!

Ruled out the fuel tank. Motor mounts, transmission mounts, wheel bearings… Have now replaced the entire rear end, axles and all, (under used car warranty) and still have the thump. Now I’m just down right curious as the whole back of the car is nearly new and my mechanic is perplexed.