I am looking for the torque specifications on the rear axle wheel bearings for 1988 e350 Econoline Van.

 

Torque spec on bearing bolts, or generally any bolts, are dependent on the grade, size and thread of the bolt. So the answer is to get a torque chart. They can be found in the back of some maintenance manuals or google it and you will get about a billion hits.
Just a caution: Bolts in special applications: Wheel nuts, rod bolts etc., are exceptions.