My truck has no reverse gear it engages.. but moves slow when i press the accelerator it goes no were just sits there ..it was slow to engage b4 but it worked fine now i have noting any help here ..it drives forward fine

Sounds like a similar problem that I had. I bet the check engine light is on. If so have vehicle scanned for codes. You most likely popped something in transmission.

Does it seem to bog down when in reverse as you accelerate? If so the trans maybe stuck in forward gear (mechanical problem) resulting in both forward & rev being applied.

transmission synchronizers most common cause of transmisson noise in certain gears, and not others, might want to check the gear oil level. Transmission rebuild required. To read about them and what they actually do, then here is a good read:
When you shift gears in your standard shift car, you move a rod that moves a fork that engages the gear. Depending which gear you're shifting to, a different fork does the job. The fork moves the collar to the desired gear, and dog teeth on the collar mesh up with holes on the gear in order to engage it. You engage reverse gear through a separate, small idler gear. The reverse gear always turns in the opposite direction of the other (forward) gears.
In years past, double-clutching was common in order to disengage a gear, allow the collar and next gear to reach the same speed, and then to engage the new gear. To double-clutch shift, you pushed the clutch pedal to free the engine from the transmission. Then the collar moved into neutral. You released the clutch and revved the engine to get it to the right rpm value for the next gear so the collar and the next gear spun at the same rate to allow the dog teeth to engage the gear. When the engine hit the right speed, you depressed the clutch again in order to lock the collar into place on the next gear.
Modern cars use synchronizers in order to avoid the need for double-clutching. A synchronizer, or "synchro," lets the collar and gear synchronize their speeds while they're already in contact but before the dog teeth engage. Each manufacturer's synchro is slightly different than the others, but the basic idea is the same. For instance, a cone on one gear will fit into a cone-shaped depression on the collar. The gear and collar synchronize their speeds thanks to the friction between the cone and collar. Then the outer part of the collar moves out of the way so that the gear can be engaged by the dog teeth.

check drain plug magnet for metal bits at it appears that the bearing may be failing and the shaft is moving away from the lock dog for the gear and not engaging properly.

Reverse is not "synchronized" forward gears are, so it does make some noise, try shifting to 2nd before engaging reverse to slow gear rotation and limit gear clash or grind.

if the fluid is to low it will not engage

It sounds like the reverse band broke. Typically with automatic transmission when you put the transmission in reverse a couple things happen. Usually, the forward clutch engages and the reverse band engages holding the reverse drum from turning, which causes the planetery gears to make the vehicle move backwards. If the band breaks there is nothing told hold that drum from turning and you have a forward gear. The bad news is the trany is going to have to come out to be repaired and usually the reverse drum is one of the first things to go into the trany, so it has to be completely disassembled. Take it to a transmission shop have them check it, hopefully it's something simpler.

Sounds like a broken internal trany part. Take it out and shift it through the gears manually to see which gears work and where it revs up. The second and fourth gears are the ones that tend to not work.

It grinds just sitting still? Will it do it with the transfer case in neutral or 4x4?

Not overly familiar with Audis but it sounds like either a.) you have an electronic tranny that has a serious bug or b.) there's a problem in the computer of the car itself. What is sounds like is the car is taking a long time to "understand" that you're telling it to move forward. How new is the car? When you push on the gas before the three minutes, what happens?