You can find torque specs by searching with the bolt diameter, thread ( fine or coarse), and grade. Also you might have some luck basing your search on the chassis/axle manufacture. And lastly, u-bolts stretch and while they are very commonly re-used, the recommendation is typically to replace them, although in this case I probably wouldn’t. If you do that you’ll be able to get torque specs on the new u-bolt.
Here’s a good starting point:
https://www.boltdepot.com/fastener-i...ed-Torque.aspx
The grade won’t be readily apparent but chances are they are grade 8. But since you don’t want to over torque, if you can’t make the grade determination, and don’t want to buy a new u-bolt, use the grade 5 spec, then test that same torque on the other un-loosened nuts. If your torque wrench immediately clicks, wind it up 10lbs at a time to find out what the others are torqued to. You’ll probably be able to get a good idea from that.