The 48RE is a beefy transmission. I did, however, have a problem that was not directly related to the transmission. At around 120,000 miles one of the six bolts that connect the torque converter to the flex plate backed out while pulling my 5er on a grade (the noise associated with that will pop your eyes open... )
I took the opportunity to have my trans upgraded to a full DTT unit with a billet input shaft and replace the flex plate w/ another stock OEM part. I felt that with a buck-twenty on the clock and having to pull the trans to get to the TC / flex plate, it was a good time to freshen up the trans with built hard parts and a nice Mag-Hytec double deep pan.
Now with 144,000 miles and the "livestock" valve body, it will still chirp the tires if I boot it a little.
Bottom line: very stout trans - doubtful that the one you're looking at has been hurt. Just drain and fill with Amsoil ATF (don't flush) and enjoy a sweet truck.