First if the problem seems to be that the transmission is, "slipping" in reverse but not other gears, there is an internal problem with the transmission. Now the fact that forward or drive has started to do this as well indicates some internal problem that would probably require a rebuild. Possibilities could be eliminated as to electronic control issues by having trouble codes scanned. This can be done at any Auto Zone or Advance Auto for free.
If I mis-understood and the hesitation is related to engine performance, the same trouble code reading will give you some direction into the cause of the problem and will be the best way to see if it may be connected to the repair.
Anytime a vehicle is repaired the possibility of damaging other parts or leaving something unhooked, like a vacuum line that was not re-attached, could be something that just happened and it is not necassarliy a sign of poor workmanship, just a mishap.
To go over the possibilities of hesitation related to engine performance, however the reverse gear being the major hesitation made me think you were talking about a transmission related problem, I am giving you a link to a walk through diagnostic on engine hesitation issues.
Here is the link;
Saturday, February 18th, 2012 AT 8:02 AM