You will have to pass through customs and immigration in order to reach your connecting flight. This is typical in the US as they do not have sterile transfers.
The Newark Airport says this about the subject:
For international arriving passengers, you must claim all checked bags
at Customs and recheck with your departing airline, even if the bags
are checked to your final destination!
The same site says that there is a shuttle bus between the 3 terminals, and that using this bus means you will not have pass through TSA security. However I don't know if you can access the shuttle after passing through immigration and customs, and dropping your bags off again.
United Airlines Operates a shuttle bus between Terminals A and C from 5:00 am to 10:00 pm. In addition, United operates a one-way shuttle bus connecting Terminal A to Terminal B and Terminal C to Terminal B between the hours of 1:00 pm and 11:00 pm (i.e., customers in Terminal B cannot use the shuttle bus to go to Terminals A and C). This service is free and you will not have to go through TSA security again. The shuttle bus is available at gate A28, B67, and C70 as illustrated in the image above. Please note this shuttle is operated at the sole responsibility of United Airlines and is subject to modification at their discretion.
1.5 hours does seem tight for this. But the same link also provides you with a link to a portal where you can monitor security and customs times. Doing so will at least give you an idea of whether you need to be an olympic sprinter in order to reach your flight, or just a normal long distance runner.