My gauge on my truck just jumps up and dont and stays on empty is it the wiring or the pump?

 

Mine isn't that bad yet but mine jumps around a bit. I think it's the fuel sending unit.

 

should i just buy a new pump or do you know if you can just buy the sending unit?

 

The ACDelco part for the sender only is #25314231. This will only work on a 2 dr Blazer (ZR2, LS, etc). I bought one from GMPARTSDIRECT dot com for my '97. Cost was $72.00 about 6 years ago.Same pump is used on the '98 to '01 2 dr Blazers. Google search will get you cheaper prices.
You have to remove the pump assembly to change it, so depending on miles for the old pump, you may be better off with a brand new pump. Dropping the tank on these things is a big chore.
Whatever you do, don't forget to get a new gasket for the tank pump assembly. It came with the part from GMpartsdirect. I'm not sure if google search/ebay parts will.

 

Mine hasn't worked for about 6 years now, works when its full and a little under full, then it goes back to full and stays untill it bounces around, then I have about 2 miles to get to a gas station. But I figured the mileage (when to stop) out fast and I just do it that way.