[DVS]

I am new to forums and don't really know how to use them yet or how to reply to an answer.

DO NOT publicly post your e-mail address, at least not in raw format like that. It just invites SPAM and scams. Sometimes people post something like:

name <at> domain <dot> com

to confound SPAMbots, but even that's not a great thing to do.

If you want someone to contact you privately, ask them to "PM" you (private message) which can be done in the "User CP" (control panel) section. That's the place where you can edit your profile, signature, etc. as well.

If someone PMs you, the next time you connect to R4W it will alert you that you have a message from someone and link you to view it.

I suggest you edit your original message to delete your e-mail address from it.

In answer to your question, your profile says you have 4WD. Your post mentions "under the weight of the car" but it also sounds like it only happens while in motion and not, for example, when stopped, because you mentioned downshifting only. As such, you might want to have the rear drive shaft universal joints checked out. It could be they're a bit worn and and there's a vibration that occurs at a specific engine harmonic.

(Though that may be a bit of a reach.)

.