91 Ranger, 4.0 automatic w/ od. old trans went out about 6 months ago, replaced with this used tranny out of a parts truck. driving home today I stopped to put it in 4wd and when I took off (in drive) it would not shift out of 1st gear. I pulled the gear selector down to (2) and it took 2nd, but when I returned it to (D) goes back into 1st and still will not shift. No clunks or slips whatsoever. checked vaccum, seems to be good. checked all electrical connections at trans, same there. fluid level good. WTF is going on here? I'm definately not a tranny expert, so any advice is apreciated.