Probably...

Is it doing it while using the cruise control??

try locking out the overdrive when you are below 55 mph.  there is a service
bulliten on this problem.  the tranny overheats due to low fluid flow while
in overdrive at this speed.  the official fix is to replace the spacer
behind the valve body i believe.  the tranny will also slip at this speed in
od when it is really cold.  one of the most common causes for this though is
improper transmission fluid.  dodge also has a cold temperature bypass kit
that helps solve the cold issue.  have you had the fluid changed recently?
michawel

Perhaps a low $ band adjustment could fix it (guessing).  If it's not
an expensive problem then it will quickly become one if you let it
continue slipping.

I had a the same thing going on with my '99 1500 4x4 at about 75k. The
torque converter went out, and the metal shavings blew out almost
every internal seal and valve in the damn thing. If it wasnt under
warrenty, the tranny alone would have been almost $900.