Whats the rumbling?

A progressive rumble felt perhaps toward front but feel it more when out on the freeway as you pick up speed. Quiets down as you slow down. A mechanic checked says the underneath isn't loose. Says he thought it might be transmission or transfer case.

The tires we had rotated to back were worn and rounded before rotating to back with the back tires now on front, in good shape. Don't see any obvious wore out on the inside as the back were. Replaced the back with new recently. A ball joint was replace on the front passenger side 1 and half years ago and immediately had alignment.

Basic maintenance oil change, spark plugs, etc, a few years ago.

Think the rumble was barely noticable when we bought it 3 yrs ago. Has progressively gotten worse, very noticeable now.

Would like to have fixed before totally breaks down. Any suggestions on what else to look for or preventive measure that can be taken and Identification of problem would be helpful-probably most of all, :).

Thank you