I have been told that I cannot use 1989 shocks on my '88.is this tru? And why? Thanks

It looks like it may depend on the model you actually have. Here is an example and see if your model crosses over.

MC Advantages :: Shocks, Heavy-Duty Adjustable, For ’89-’99 Softail Chrome (http://www.mcadvantages.com/shocks-heavyduty-adjustable-for-8999-softail-chrome_p_-1,1043,53,201,344_446.cfm)