you'll first need to figure out whats going on for sure.if she's dropping a gear,down into 3rd or if the converter is unlocking.more than likely the converter is unlocking rather than a gear change.
is the od light on the shift handle flashing? if so,did you get the codes yet? when this happens at these speeds,do you notice a flickering of the speedo or tach just before it happens?
if this was happening to my trucks id for sure figure out first if it was the converter or a gear change (a gear change would be much more than just a couple hundred rpm difference.) then get the codes but if i noticed a flickering speedo,well if i did or not,the PSOM and the VSS would come instant first suspects to me given it's between 45-55 mph (im assuming all is fine outside,one way or the other of these speeds,but you know what you get for that lol.)