Tensioner bad?
 

The vehicle is a 1990 350SDL (without bent connecting rods).

Low rumbling starting in the front of the engine. Stops at acceleration. Figured it is the serpentine belt tensioner shock bushings starting to go even tho, they have been replaced about 8,000 miles ago.

I went to look at the shock and tensioner assembly and noticed that the tensioner is NOT parallel with the belt. Is this a problem I should be worried about in the future or is it normal? I never noticed whether it was parallel before or not.

If the tensioner is not parallel with the belt, CHANGE IT ASAP. Replace the belt shock too.

Seeing as how you are about to drive around 1300 miles, and probably do not have time to fix it, I would grab a spare belt for along the way, and what ever tools you might need to change the belt. Seems kinda weird that it is doing that. I wonder if its the bracket, or that the tensioner wheel itself is crooked.

On the diesel OM60x motors, if the belt tensioner breaks, the entire timing case can get torn up and the engine is more or less done for. You will be screwed. Get that taken care of ASAP - especially before you take that road trip.

Quoted from mbz.org:

"...check that the serpentine belt idler pulley (right below the horizontal expanse of fan belt at the front to of the engine) MUST be aligned parallel with the belt - if it's a degree or two cocked, then it MUST be repaired..."

Note the emphasis on MUST. :)

Repair it or have it repaired before you go. It will take much less time now than later when you have to call a tow truck, find a competent shop, and them get the parts if they dont have them in stock. I have seen this many times and it will not last that long.
Brian