Advice on fixing this, please.

[whocares]

I have an 89bII with the stock 7.5, ls, 3.73. I was under the truck today checking my u-joints and noticed the rear pinion has some movement to it when I move the driveshaft up and down.
Its quiet on the road, no whine, leaks or noise at all from the rearend, is it possible I can get away with just tightening it or am I looking at something more involved?

[4x4junkie]

You can try retightening it to around 120ft-lbs or so (don't go more than this though, or you'll likely be replacing the crush sleeve), though simply not being noisy isn't a guarantee there's a bigger issue. Do put a very tiny tack-weld on the nut after you do tighten it though so it can't loosen up again (alternatively you could remove the nut entirely, clean all the oil off the threads with some no-residue spray cleaner, and reinstall the nut using Loctite threadlocker).

Be sure to check that the pinion bearing preload torque is within specs after you tighten it (you'll need to remove the diff carrier to do this correctly). Specs can be found here. If it feels rough at all, chances are good the pinion bearings are hosed.

[whocares]

I appreciate the advice. I'm hoping I can tighten it and it will last until I run across the right deal on a 8.8, I really dont want to spend the money and time on fixing the 7.5.