I have a problem with my transmission, the 3rd gear synch. has gone out, so would it be cheaper to replace the tranny, or to tear it down and pay the 400 dollars for the new synch?

Find out how much you can get a GOOD used trans for and compare that to the price to get yours fixed. You could also double clutch it into third and keep running it as is.

I have been doing that, but I have a problem downshifting with a load, but I will check on some used tranny's, thanks.