Most likely it is the gearing/tire size. Look up some ring and pinion sites,get a phone number and call them. tell them what you are running and they will tell you the proper gears that you will need.
Also the 700r4 is a great tranny as long as it is 88 and newer,the older ones had a problem that they fixed in the newer ones.I think there is a kit that you can get that will fix that problem. Call up you local GM dealer and find out if there is a kit,if not see if you can trade in the older tranny for a newer one.Or call a tranny shop and see what your options are. personally i would go with the second choice,most dealers dont rebuild trannys anymore it is just a remove and replace.the trans shop will know more about what can be done to your trans to get the most from it.