You can tell what trans you have by the shape of the pan; check the LMC truck site here
http://www.lmctruck.com/icatalog/csb/t12.html for an explanation. The 700R4 (which you most likely have) has an overdrive; going to a different model will cost you fuel mileage but I have heard it can be done. Have you checked your "kick down" cable; I understand it is VERY important to the proper operation on the 700R4. Make sure it is properly adjusted and not broken or disconnected.