I would wait until you install the O2 sensor and see what your mileage does, if it comes up, you were probably running rich, if it stays the same, the cat may have just went bad, though I would think for the price of the Magnaflows, they should last longer than 4 yrs.

Did the shop ever take a pre-cat reading, this would have told you how well the cat was working or wasn't, and would also indicate if it was running rich.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed