U joint would be my guess too.
Below is what Mercruiser has to say about running in "Trailer" mode. It is pasted right out of the operators manual.
"Power Trim allows the operator to adjust the drive angle, while
underway, to provide the ideal boat angle for varying load and water
conditions. Also, the Power Trim system “Trailering” feature allows
the operator to raise and lower the drive unit for trailering, beaching,
launching and low speed (below 1200 rpm engine speed), shallow
water operation.
CAUTION
Never trim the drive unit UP/OUT using TRAILER switch while
boat is underway at engine speeds above 1200 rpm. Use extreme
caution when operating with drive unit raised. Severe
damage to the drive unit may result if unit is raised beyond the
gimbal ring support flanges at engine speeds above 1200 rpm."
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