Hard thunk on downshifts from 4th to 3rd...AW4, 1999...trans or mechanical issue?

Changed the fluid in the 99 sport tonight (and the filter) Solenoids test fine....Dex/Merc.

Changed the fluid a week ago...roughly 4 qts each time...brown the first time, less the second.

Daughter went for a test drive, said it seems improved but still happening..
(will know better tomorrow on her regular drive to work..).

I went out with her...and (apparently it's pretty predictable when it's going to happen)...there's a solid thunk when she's moving at a slightly slow speed in 4th...and gives it just a bit of gas to get it to shift to 3rd..

Feels like it's about where the rear seat is...(at least behind the driver/passenger).

Could something else cause the thunk...bad Transfer case, driveline U-joints, rear diff issues?

Shifting up is smooth...barely can tell as it shifts..
But it's that 4 to 3 shift that's when it happens...

Any suggestions?

Thanks!

Due to the location of the 'thunk', I'd first
grease the slip-joint at the transfer case.

That cured mine, although mine did it when
starting from a dead stop...

As suggested, grease the rear axle shaft slip yoke. While you are there, inspect the u-joints. Inspect the motor mounts, transmission mount, and the exhaust pipe hanger mount at the crossmember. Inspect the exhaust pipe for signs of contact with crossmember or unibody. Inspect the leaf spring bushings.

Thanks for the info. I'm going to try that (this afternoon if possible), and do the inspection on the mounts/u-joints.