Although the transmission shifts just fine up and down, it makes a big clunk every time. I am afraid one day it will make its last clunk. Is this something that I can fix myself, even though I have two left hands?

Do a search on shift modulator and check the vacuum line that goes to it.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma