1995 Chevy Saburban transmission stuck in 3rd gear

1995 Chevy Saburban, currently at 150,000 miles, Prior owner had transmission rebuilt prior to 100,000 miles. While towing my 18ft camper trailer on a recent trip every time I started off from a stop I had to really push on the gas to get going. At first I though the emergency brake was on, or I had a flat tire. I then noticed the automatic transmission was not shifting when I had it in D. Checked the transmission fluid and it was full. The transmission seems to be stuck in 3rd gear. When I move the shifter to 2 it will shift down. Reverse is also still working. Am I in for a costly repair?

Check out your previous post (now on page 3). Transman618 left you a couple of suggestions and is the resident expert on GM transmissions and control systems.

Thank you, Thank you