93 Suburban

Hello I have a 93 suburban 350 auto 158k the problem is the Transmission it was rebuilt by a reputable transmission shop that does good service, 8000 miles ago it had to have three separate trips after the first rebuild which were all covered under waranty. Now it intermiitently will actually shudder the entire truck seems to vibrate when shifting it doesn't happen all the time but it seems to happen more frequently when towing a load. The shop who completed the rebuild seems to think it is an electrical problem with the trailer wiring harness I have replaced this but still have the same problem. Any ideas or am I going to have to replace this transmission I am tired of this thing constantly failing.