Replacing the temp gauge isn't all that difficult, just time consuming. I believe your 88 has the same style dash as my 86. Once you get the trim cover and faceplate removed then the gauge itself is just a plug in. You can search local wrecking yards for one or you could check
http://car-part.com/ or get a new one from LMC truck at
http://www.lmctruck.com/icatalog/csb/t1.html. Good luck!