My 1984 300TD wagon is "slipping" between 2nd & 3rd. On a straight away, it's not a big deal, but when going up a steep hill, the car starts slipping and won't shift during the gear change.

My question is, is it possible to shift to one of the 2 lower gears while the car is in motion? I have stopped, put the car in the lower gear and started and this keeps it from slipping, but, never tried it while moving. Don't want to dump my already suffering tranny!

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1984 300TD Wagon