Vibration in driveline

[nspctr1]

When backing up the 998 and I apply the brakes, I get a strong vibration. feels like it is coming from the rear disc but not sure. It doesn't do it while going forward which seems kind of weird to me. Is it normal? How can I diagnose it, fix it? It's a 1992 with the 6.2 in case that matters.

 

[Bulldogger]

It could be a lot of things without more information. But I would check to see if one of your hubs maybe loose. Jack her up at each tire and see if there is any play. At the same time you can rotate the tire and see if there is too much play in the jack shaft.
Bulldogger

 

[86humv]

There are brackets that hold your diff on the side.....the big bolts back out sometime.