They can do this for many reasons, from worn uni joints the bad engine mounts. If the transmission has not been serviced for a long time then maybe it would be adviseable to get it serviced at a reputable trans specialist - tell them the problem and they will locate it for you - they have the advantage of getting the car on the hoist. It should not require a new transmission!!! If you are mechanically minded you could do some investigation - like eyeball the front and rear engine mounts - check they are not oil soaked or split.
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