I have a 1999 GMC Envoy. I started it up yesterday and took it for a trip. The transmission always shifted a little hard from 1st to 2nd and occasionally threw a P1870,(on long trips) but now Usually but not always, now as soon as i step on the gas and get to about 3 MPH it hesitates and shifts into 2nd pretty smooth.It also has very poor acceleration, I cant get the RPM's above around 2500, even punched on the freeway. Also all the sudden all lights on the 4X4 switch are on and its in 2WD. The 4X4 used to work just fine! Now there is no CEL ( I scanned it multiple times). Also only getting about 10.1 MPG. Sounds like a tranny solenoid/ valve body/ TQ or maybe TPS sensor? not sure give me some ideas!!

 

The valve body had issues prior to 2000 on some trucks causing the trans to slip horridly. You may want to find a junkyard 2k+ valve body and swap out all your solenoids and trans filter while your in there.