Had my trans rebuilt in my 2001 Dodge Ram 1500 4X4 20,000 miles ago. Yesterday i took notice that it is slipping in reverse. All the forward gears seem ok. So what are the possible problems with it? Hopefully it is something simple!!!

Harold there is a valve in the cooler line that goes bad and can produce what is going on at least that is what happened in my '03 Ram 2500 by the time i needed to go forward it was already getting fluid.....

You owe the rebuilder a phone call.

Why diagnose what he should repair ?

Already called the builder just trying to get some idea's because he will fix it on my coin because with 20,000 miles it is out of warranity.

I’ll assume it is an overdrive trans.

Take the truck for a ride in manual low gear, accel up to about 3500 then tip out of the throttle and see if you have engine braking.
Do it again in manual second and see if you have engine braking.

Report back.

PS - just my opinion, you should take it somewhere else if he is going to charge you without knowing what is wrong.

Most good rebuilders warranty their work past 20,000 miles.

Around here, we get one year and/or 12,000 miles warranty on a rebuilt trans. You can buy an extended warranty, but by the time you get much past the 2 years and 24,000 miles, you might as well just take your chances, the warranty is nearly as expensive as a rebuild.
Almost no one is rebuilding transmissions around here, everyone is installing one of the reman company units. You get the warranty from the reman company, and the local guys still charge you R & R on a warrantied trans..

And thank you transman for always trying to help

All true.. Rebuilding has become less than cost effective for most modern transmissions.

I rebuilt a GM 4T65E a few years back. By the time I replaced all the defective parts I was $200 past the cost of a rebuilt unit.

I was asked to rebuild a Ford RWD transmission and the basic kit was over $1200. That was just the kit. No parts like solenoids or TQ. It wasn't worth it.

Old school can still be done cheap but modern stuff is not cost effective.

So many scenarios to consider.... is this like first start in the am? Like jump in start engine put in R to back up? Does it happen warm or cold? Happen any time R is used?

Did you google "Tranny" for possible help? After you get through the sick pages there are common problems mentioned.

Don't get advice from Caitlyn Jenner.

...................

Transman i did what you said to try in 1st and 2nd and it does hold back when leaving off of gas!!!!

Then the overdrive unit is not at fault.

When you say slipping in reverse, does that mean it always slips or does it mean it delays then engages ?

Does it go away after warmed up?

Need more info.

exactly, start the truck up in neutral instead of park. Allow it to run at least 5 minutes for test purposes.The most common thing is the fluid runs out of the convertor and pump over night loosing its prime. Neutral allows it to gain the prime back. Then put in reverse and see if it still slips. Birdtracker

So today i ran it in neutral for 5 minutes and to no avail. When backing up weather hot or cold it slips but as you go back for awhile it begin's to stop slipping.

so it is not loosing its prime. Thats a good thing! Its odd that it doesn't slip in low gear going forward as they share the same drum only it spins the planetarys the opposite direction. If it was mine I would pull the pan and check the band adjustment. Replace the fluid and filter also while you are there. I had a 2003 or 2004 Dakota my neighbor brought me. He drove it in. Had me fix a few things and wanted the trans serviced. I pulled the pan off and the band material looked like a big rats nest in the pan. I was totally shocked that it still drove and shifted as it should.Birdtracker