1999 Suzuki Grand Vitara when should I replace shocks &

IF they are original it would certainly make a difference in your ride. Weak ones will cause an abnormal wear pattern

The vibration may be from the irregular wear or tire balance.

If the uneven wear is edge wear, then it may be from a lack of rotation or an alignment issue.

You can do a bounce test on the four corners of the truck. IF you get a few good jounces on the bumper the truck should recover after one bounce. IF not the struts/shocks are weak.

IF they are leaking excessivley, then replace them.