Try the easy/cheap stuff first. First thing I'd check is the linkage, just to make sure it's not actually in low range when you think it's in drive.
Then check the vacuum modulator. It's on the back of the transmission, I believe on the right-hand side. There's a pic below. It should have a vacuum line attached, which runs up to a vacuum source on the intake manifold. If the modulator is defective, or if the vacuum line is leaking, the trans will only shift out of first gear at high rpm, no matter what position the throttle is in. The line should be metal, with rubber connections on both ends, but sometimes they get replaced with a long rubber tube, which might collapse with age and heat.
The straight part of the above modulator will be inside the transmission, and the hose connection may be at a right angle compared to the one pictured.