81 chevy converter lockup

[75gmc454]

I have been looking for a short bed silverado with a 6 cylinder and air conditioning for a while now. Well the other day I found this truck for sale and bought it. It has been sitting for about 5 years so I changed the oil and filters and got it running. It still needs a lot of cleaning though. Well as soon as it shifts into high gear the torque converter locks also, causing a big drop in rpm. I didnt think it should lock up until at least 40 or 45? Any ideas on how to fix??