The weak point on the ZF 5-speed is the synchros. Mainly because they last forever otherwise...
All the 5-speed trucks I have tend to be a little 'finicky' going into second(you either have to be going really slow to downshift, or rev-match), and several of them 'clash' a little going into 3rd - you can feel it bump a couple of teeth sometimes.
Reverse can be a little iffy as well; once you get the feel for it, it's not a big deal, but it's not hard to put it in reverse... and find out the transmission isn't /in/ reverse.
It's all due to worn synchros... but considering the transmission probably has 240-300+ thousand miles on it, that's reasonable.
Even if your truck has *all* of these issues, the transmission isn't going to go out any time soon. You either A, rebuild it or B, ignore it and learn the
'feel'. I've put 40K on a truck that had all of those issues... and it still has them the same as it did 40K ago. It didn't get worse, or cause me issues or anything.
Also, while the 5-speed ZF transmission in your 1990 is a zf s5-42, meaning it's rated for 420 lb-feet of torque... Multiple people have run more through them. Sometimes a /lot/ more. IIRC, Justin @ R&D was putting over 1,000 ft/lbs of torque /at the wheels/ through a zf s5-42 before he blew a connecting rod, which means the transmission was handling more than that...