I was told the balance shaft could possibly be bad in my 1995 Buick Riviera Suoercharged car less than 54k miles. Harmonic balancer & motor mounts replaced. Expensive repair?

What type of problem(s) is the vehicle experiencing?

I had hope to get a response..but I will offer this opinion/thought. It appear from your statement that the vehicle is vibrating. I too experienced that with my 95, changed everything you have and it was suggested the problem could be balance shaft. Had that checked and NO it wasn't that. The problem may lie in the fact that GM in 95 had two different harmonic balancers for that engine. If by chance you have the original balancer look at the backside/inside and there you will find a bar/plate. The different is the size of this plate/bar. On is larger than the other. Good Luck>>

And too there are 6-8 different part no.'s The following is the one that solved my problem. Part Number: 88960264.

The 95-99 Riviera crankshaft is balanced from the inside. The crankshaft has to be removed to be balanced. I had the same problem.

I had a 96 that had similar problems ... electronics was the issue in my case (and continues to be the issue now ) ... The ICM was replaced after all other stuff failed and that solved the problem ... although it seems to burn up on a regular basis .. about every 18 months or so .. i always keep a backup in the trunk ..weird I know ... I've replaced all electrical parts as well ... .