1990 Cougar tranny needs answers

1990 Merc Cougar 190K miles, 3.8 L auto. Tranny has been rebuilt for the second time, first rebuild lasted about 85K miles. After they did the job they told me to drive om D in town and use overdrive for the expressways only, told me that it will lessen the load on the tranny and prolong it's life. I've never heard of that before and thought that on the more or less flat terrain one should always use overdrive unless you are pulling load or something like that. So does anybody can tell me if there is any truth to that or they are just covering up a questionable job. I have some rattling on the back of the driver's side (not sure about passenger's side yet) that was not there before and excessive vibration on the front end also. To top it, driving on D kills my gas milege. What do you guys think?
Thanks. Irina.

 

They are full of it. If they correctly rebuilt your tranny, you should not have to do this.

99% of people never turn their OD off, most of them don't even know how.

They are trying to avoid fixing their mistake. Have another mechanic/tranny specialist take a look.