2.8 S10 wont start or turn over

i have a 1987 2.8 TBI s10 automatic and just changed the plug wires and spark plugs a couple weeks ago right when i got the truck from a friend (free). well i went out to start the truck right after i did the tune-up and it started fine but as soon as i put it in D it completly shut down and i tried and tried to start it again but no go. a day later i went out there and tried it again and it fired right up. but yet again as soon as i put in D it died. well when i try to start it it doesnt crank and the starter doesnt even engage or "click". after tinkering with it for awaile i put my foot on the brake and shifted through the gears P-1st and put it in N and it started right up and then i put it in D and i took it around the block and it never missed a beat. well i took it out again and as i was driving down the road it just DIED, i lost everything, power steering, power brakes and it wouldnt start no matter what i did. also it is leaking a bunch of oil from the engine to the tranny and i went to check the oil level and it was fine, looked at the starter and had fresh oil dripping off it. my dad said it could be a relay gone bad or the oil pressure sending unit gone bad. also all the fuses are good as far as i can tell. i dont know much about s10's as it was just given to me. im a LS1-LS6 guy and am at a loss right now. thanx for any responces in the future.