I'm getting hard clunking shifts from my tranny after doing extensive work around my engine bay (head gasket and timing cover work). My flex couplers are taking a beating!

I suspect I broke a brittle vacuum line somewhere ... possibly the Auto tranny modulator line. Can someone please tell me where this line is located in the bay? Or any other possible causes for this sudden change in shifting?

TIA!

~Paul

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'91 300E, 208k miles