Worth just checking your transmission fluid - there is a heat exchanger in your radiator which can leak internally, hence cross contamination of coolant and trans oil.
Most likely head gasket though. Hope so for your sake - it's cheaper than a rebuilt trans!
Adrian

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90 300TE 24v (M104 eng) 207K miles
87 300TE (M103 eng) 269K miles (and never had the head off!)
86 300TE (M103 eng) 230K+ miles (donor car)
95 Toyota Lucida diesel 4wd
86 200T 165K miles (sold)