99 Safari Tranmission Meltdown

I recently had the transmission in my 99 Safari rebuilt and the reason given for the meltdown was that there was electrical current running through the bearings possibly due to improper gronding. The fix was to add a ground strap to the trans.

I do not distrust the mechanic, but the explanation given by his trans rebuilder seems questionable. I believe battery current could melt a bearing but I had no obvious electrical symptoms. Also, the trans is bolted to the engine and I do not see how adding a groundstrap would stop the flow of current through the bearings.