1995 chevy truck 1500 2x4 transmission problems?

I have a 1500 2x4 chevy truck with 179.000 miles, my transmission started to act up with me, now it won't change automatic from first gear i have to put it in 3 gears to move, i took to one mecanic and he told me it's the transmission, i have to rebuilt, and i took to anothe and he's telling me it's a eletrical problem, the speedometer dont work, a it change gears manually, even the reverse, What could it possibly be..??? PLEASE HELP ??