The '84 would have been equipped with a 3.07 diff from the factory. The '85 is equipped with a 2.88. The difference is slight and the later diff will result in slightly slower off the line performance and slightly slower rpms on the highway. The top speed of the vehicle cannot be affected in any significant way by this change.

My guess is that the mechanic dislodged the pressure line to the ALDA while working under the hood for some reason that you have failed to mention.

None of the issues described in the post can be attributed to a diff change.