Did you check transmission fluid levels?
Also at that mileage it usually needs a lot of work. Unless the body, interior is spotless, great tires and good overall shape this sort of machine is rarely repaired. The owner of one this old and not in good shape may be doing the old "just get it running for another few months" routine.
Check fluids and while you are in there get out the flashlight to check for unplugged, cracked vacuum lines. I've lost count of those issues.
My 2006 Tahoe (289,000 miles) started making a whining/shuttering noise a few weeks ago. The noise only happened when accelerating or stopping. Yesterday the truck started, and went in reverse, but would not move in drive. Can anyone give me an idea of what the problem may be? I don’t want to pay to have it towed somewhere just to be told its the transmission, which I will not get replaced on such an old vehicle. Any help, would be appreciated!