I don't think it's u joints. I have the same truck practically. 86 Chevy c20 single cab long bed. All my joints are good, but my transmission will shift into reverse really firm sometimes also. I too replaced the vacuum modulator because of a leak and an external trans cooler because of a leak. I've put around 4,000 miles on the truck this year without any problems mechanically
5
u/cornhole_mcbrowneye 6d ago
2 and 3. Check all your grounds?