"Thumping" from rear of vehicle- Help

[Pa Jigmaster]

98 4.0 SOHC Auto with C-Trac transfer case, 100,000 miles - I've done several searches on this problem but I'm not sure if any of the causes fit my situation.
While driving at moderate loads and speed, a series of loud thumps/clunks is heard/felt coming from the passenger side rear or maybe just the rear. Sound is intermittent and can be one time or four-five times in a row. FORD checked it and wasn't sure. Local mechanic had rear diff. cover off and didn't find anything major. He said thumping diminished greatly when driving in
4-hi. He thinks it is the transfer case but would that show up as a sound in the rear of the vehicle?? Also, under hard acceleration or on steep uphill, thumping is not noticeable. Problem is getting worse. Thanks for your help.

 

[boominXplorer]

Check your driveshaft bolts! Same thing happened to me and i couldn't figure out. Look right where the driveshaft goes into the t-case. Mine was looked over by 2 shops then i found it while i was changing oil.

 

[Pa Jigmaster]

Thanks Boomer, I will check that again. Most of the possible loose stuff was checked but something could have been missed.
Also, a good description of the "thumping" is like somebody hitting on the hatchback floor with a rubber mallet.
Any other thoughts?

 

[AlaskanJack]

I was having similar problem then it became more regular. Thought it was t-case swapped it out but sound remained. Finally put all 4 wheels on stands and let it run in drive. Sound was from rear. Friend assisted me and it appears to be pinion output. Pinion Nut was loose but it turned out I had also busted 4 of the pinion teeth.

But if they pulled the diff cover and looked you should be good.

 

[sean4385]

I believe the proper phrase-ology would be "knocking."
If thats the case then it could be shock bushings.....
dont ask me what that means but my friends blazer has that problem. but thats only when he hits bumps, i dunno

 

[Pa Jigmaster]

AL-Jack - Good idea on running it on jackstands. Will suggest that to mechanic.
Sean - "Thump" happens on smooth roads intermittently.
Almost sounds like a loud secondary explosion/bad miss traveling back thru the exhaust system but mechanics(3) say not so.
Thanks guys. Any more ideas???