Differential clunk noise in reverse

[norske]

Lifted the bed off my 68 Chevy c10 and decided that's the best time to change the pinion seal, change the differential fluid, and check u-joints. The pinion seal appered to be leaking, so replaced it. All the u-joints were no good, put in all 3 of them. It's a 2-piece driveshaft with the carrier bearing (that was ok). The rear end is a 12-bolt with 8 7/8" ring gear, says GM56 on it. Talked to a guy who works on the local dirt track circuit and he said there was no preload on the pinion nut, just rattle it back on. When I got finished, backed it out of the shop and noticed the rear end (I think) seems to bind and "clunk" in reverse. It doesn't do it going forward. I don't know if it did it before or not. There was just the nut and washer on the pinion. Could I have done something wrong (There is a preload on the pinion nut?) or maybe a problem that I just didn't notice before? Going to jack it up and spin the wheels and see what's up. Any ideas appreciated.