Why did my Pickup go 'Haywire' after new Battery Installation.

Help! I Had a 'Diehard' Battery installed last night in my 98' Ford
Ranger automatic, however, when I started to drive off after service was
completed and paid for, my truck wouldn't get out of first 'gear', my
'o/d' (overdrive) light started flashing off and on for a short period,
there was definite hesitation in acceleration, and my speedometer
stopped working as well as my interior light when I tried to turn it on.
EVERYTHING was operating perfectly beforehand, and the 'Sears' guys
didn't seem to have a clue when I drove it back to question this
situation. Now my Truck hesitates big time like it's stuck in gear or
something, and I'm sure this was not a coincidence, WHAT HAPPENED???
Someone told me about 'codes' being reset or somthing......WHAT???