1994 Buick park avenue has 60,000 miles and now won't shift into high gear.

Hello, When the transmission starts leaking, the low fluid levels will cause erratic operation. After 23 years on the road, I'm surprised this is the first repair the transmission needs. If the leak is the only issue, repairing the leak and changing the fluid and filter is all you will need. If the shop that did the service also serviced the transmission, it is likely they over tightened the transmission pan.