I've been through the whole transmission, rebuilt the valve body, checked the band servos and set the bands but the damn thing just slips although I got it to lurch forward for once but if i take one tire off the ground I can get it up to 80Mph and all the gears shift correctly....help!

 

just a thought, have you checked to see if your driveshaft is spinning when it's on the ground? It could be the rear-end?

 

So, you've checked the brake band servos, set the brake bands, presumably correctly, but it still slips in the gears. However you don't say what car it is, an 87 could be an XJ6 Series 3 or an XJ40, (the next model up). Is this a Borg-Warner Model 66 transmission ?

 

The car is a 1987 Jaguar XJ6 Vanden Plus 4.2L with the borgwarner 66

the car works great in reverse but wont go forward although when I lift one rear wheel off the ground I can get the rear wheel to start spinning forward I was able to get it up to 80 MPH! although the transmission was slipping bad but it was shifting just fine, but when I put the car back on the ground it can't even move under its own weight...it just sits there and slips.

this is what happened..
I drove the car on a 70 mile round trip when I got home I parked it everything was just fine, the next day I went to go drive it the next day I backed out of the driveway and rolled down the hill looking at my cellphone not realizing that the front moving gears weren't working as I went to get on the highway and it just rolled out into the middle of the road.

 

I think im just going to do a T350 conversion on this seems the transmission is a goner

 

Too bad you're so far away, I'm parting out an 87 XJ6 with a good tranny!

 

I will pay for UPS freight shipping how much do you want for the transmission? or email me pmally (at@) gmail.com

 

It's still in the car but I can fix that easily enough. Make me an offer? I'm a reasonable man and willing to help a fellow enthusiast!