94 LeSabre Transmission Question

My 94 LeSabre with only 289,000 miles started thumping and shaking like it was missing while cruising at 75 mph. I was able to finish my trip, about 150 more miles with occasional jerking, but not as bad as the first time it happened. My mechanic read a code of P0740 and said it was a solenoid in the transmission if I understood him correctly. I am still able to drive the car around town with only occasional missing or slight shaking. My question is what is the worst I can expect while driving this around town. I don't want to spend the money to repair it with this many miles on it.
How long can I expect to drive it with this code on? Anyone have any ideas?

No real way to answer this, just go till it won't.